Fix UnboundLocalError if preprocessing returns an empty list

If this tokenization function is used with IterableDatasets and no sample is as big as the context length, `input_batch` will be an empty list.
```
def tokenize(batch, tokenizer, context_length):
    outputs = tokenizer(
        batch["text"],
        truncation=True,
        max_length=context_length,
        return_overflowing_tokens=True,
        return_length=True
    )
    input_batch = []
    for length, input_ids in zip(outputs["length"], outputs["input_ids"]):
        if length == context_length:
            input_batch.append(input_ids)
    return {"input_ids": input_batch}

dataset.map(tokenize, batched=True, batch_size=batch_size, fn_kwargs={"context_length": context_length, "tokenizer": tokenizer}, remove_columns=dataset.column_names)
```

This will throw the following error: UnboundLocalError: local variable 'batch_idx' referenced before assignment, because the for loop was not executed a single time

```
for batch_idx, example in enumerate(_batch_to_examples(transformed_batch)):
     yield new_key, example
current_idx += batch_idx + 1
```

Some of the possible solutions

```
for batch_idx, example in enumerate(_batch_to_examples(transformed_batch)):
     yield new_key, example
try:
     current_idx += batch_idx + 1
except:
     current_idx += 1
```

or 

```
batch_idx = 0
for batch_idx, example in enumerate(_batch_to_examples(transformed_batch)):
     yield new_key, example
current_idx += batch_idx + 1
```

Comments:
_The documentation is not available anymore as the PR was closed or merged._

<details>
<summary>Show benchmarks</summary>

PyArrow==8.0.0

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.009286 / 0.011353 (-0.002067) | 0.005478 / 0.011008 (-0.005530) | 0.109768 / 0.038508 (0.071260) | 0.088460 / 0.023109 (0.065351) | 0.387664 / 0.275898 (0.111766) | 0.457379 / 0.323480 (0.133899) | 0.006517 / 0.007986 (-0.001469) | 0.004037 / 0.004328 (-0.000292) | 0.083911 / 0.004250 (0.079661) | 0.071658 / 0.037052 (0.034605) | 0.385065 / 0.258489 (0.126576) | 0.460928 / 0.293841 (0.167087) | 0.048062 / 0.128546 (-0.080484) | 0.016343 / 0.075646 (-0.059303) | 0.373675 / 0.419271 (-0.045597) | 0.067640 / 0.043533 (0.024108) | 0.391730 / 0.255139 (0.136591) | 0.432908 / 0.283200 (0.149708) | 0.035748 / 0.141683 (-0.105935) | 1.767625 / 1.452155 (0.315471) | 1.965606 / 1.492716 (0.472889) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.277405 / 0.018006 (0.259399) | 0.538448 / 0.000490 (0.537958) | 0.013795 / 0.000200 (0.013595) | 0.000518 / 0.000054 (0.000464) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.043962 / 0.037411 (0.006550) | 0.115305 / 0.014526 (0.100780) | 0.117572 / 0.176557 (-0.058985) | 0.182168 / 0.737135 (-0.554968) | 0.114833 / 0.296338 (-0.181505) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.604209 / 0.215209 (0.389000) | 6.186113 / 2.077655 (4.108458) | 2.771067 / 1.504120 (1.266947) | 2.425420 / 1.541195 (0.884226) | 2.475200 / 1.468490 (1.006710) | 0.887096 / 4.584777 (-3.697681) | 5.214349 / 3.745712 (1.468637) | 4.989606 / 5.269862 (-0.280256) | 3.092135 / 4.565676 (-1.473541) | 0.104464 / 0.424275 (-0.319811) | 0.008994 / 0.007607 (0.001387) | 0.732819 / 0.226044 (0.506775) | 7.396007 / 2.268929 (5.127078) | 3.371167 / 55.444624 (-52.073457) | 2.645475 / 6.876477 (-4.231001) | 2.704215 / 2.142072 (0.562143) | 1.034724 / 4.805227 (-3.770504) | 0.219063 / 6.500664 (-6.281601) | 0.073863 / 0.075469 (-0.001606) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 1.625020 / 1.841788 (-0.216768) | 23.369980 / 8.074308 (15.295671) | 22.480951 / 10.191392 (12.289559) | 0.228219 / 0.680424 (-0.452204) | 0.026981 / 0.534201 (-0.507220) | 0.487670 / 0.579283 (-0.091613) | 0.582310 / 0.434364 (0.147946) | 0.539182 / 0.540337 (-0.001156) | 0.791962 / 1.386936 (-0.594974) |

</details>
PyArrow==latest

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.008657 / 0.011353 (-0.002696) | 0.004971 / 0.011008 (-0.006037) | 0.089499 / 0.038508 (0.050991) | 0.075963 / 0.023109 (0.052854) | 0.497719 / 0.275898 (0.221821) | 0.507912 / 0.323480 (0.184432) | 0.006067 / 0.007986 (-0.001919) | 0.004118 / 0.004328 (-0.000210) | 0.079397 / 0.004250 (0.075146) | 0.059181 / 0.037052 (0.022129) | 0.501108 / 0.258489 (0.242619) | 0.565792 / 0.293841 (0.271951) | 0.048818 / 0.128546 (-0.079729) | 0.014813 / 0.075646 (-0.060833) | 0.093863 / 0.419271 (-0.325409) | 0.060824 / 0.043533 (0.017292) | 0.489289 / 0.255139 (0.234150) | 0.533624 / 0.283200 (0.250425) | 0.034997 / 0.141683 (-0.106685) | 1.770574 / 1.452155 (0.318419) | 1.837213 / 1.492716 (0.344496) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.237319 / 0.018006 (0.219313) | 0.594976 / 0.000490 (0.594486) | 0.008888 / 0.000200 (0.008688) | 0.000124 / 0.000054 (0.000070) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.036955 / 0.037411 (-0.000456) | 0.097825 / 0.014526 (0.083299) | 0.111139 / 0.176557 (-0.065418) | 0.174776 / 0.737135 (-0.562359) | 0.117755 / 0.296338 (-0.178584) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.606498 / 0.215209 (0.391289) | 6.089874 / 2.077655 (4.012219) | 2.811135 / 1.504120 (1.307015) | 2.428486 / 1.541195 (0.887292) | 2.399512 / 1.468490 (0.931022) | 0.823492 / 4.584777 (-3.761285) | 4.897107 / 3.745712 (1.151395) | 4.407589 / 5.269862 (-0.862272) | 2.868442 / 4.565676 (-1.697235) | 0.098774 / 0.424275 (-0.325502) | 0.007998 / 0.007607 (0.000391) | 0.699489 / 0.226044 (0.473445) | 7.139214 / 2.268929 (4.870285) | 3.511158 / 55.444624 (-51.933466) | 2.775459 / 6.876477 (-4.101018) | 2.951549 / 2.142072 (0.809477) | 1.006921 / 4.805227 (-3.798306) | 0.200105 / 6.500664 (-6.300559) | 0.071064 / 0.075469 (-0.004405) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 1.680599 / 1.841788 (-0.161189) | 23.399777 / 8.074308 (15.325469) | 21.776357 / 10.191392 (11.584965) | 0.264697 / 0.680424 (-0.415726) | 0.034272 / 0.534201 (-0.499929) | 0.506984 / 0.579283 (-0.072299) | 0.609556 / 0.434364 (0.175192) | 0.599014 / 0.540337 (0.058677) | 0.824068 / 1.386936 (-0.562868) |

</details>
</details>

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