Fix missing info when loading some datasets from Parquet export

Fix getting the info for script-based datasets with Parquet export with a single config not named "default".

E.g.

```python
from datasets import load_dataset_builder

b = load_dataset_builder("bookcorpus")
print(b.info.features)
# should print {'text': Value(dtype='string', id=None)}
```

I fixed this by setting the default config name when there is only one config.

Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_6635). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

<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) | 0.005577 / 0.011353 (-0.005776) | 0.004452 / 0.011008 (-0.006556) | 0.067849 / 0.038508 (0.029341) | 0.032328 / 0.023109 (0.009219) | 0.256924 / 0.275898 (-0.018974) | 0.273410 / 0.323480 (-0.050070) | 0.004359 / 0.007986 (-0.003626) | 0.003484 / 0.004328 (-0.000845) | 0.053880 / 0.004250 (0.049630) | 0.058142 / 0.037052 (0.021089) | 0.268863 / 0.258489 (0.010374) | 0.307977 / 0.293841 (0.014136) | 0.028840 / 0.128546 (-0.099707) | 0.011808 / 0.075646 (-0.063839) | 0.216277 / 0.419271 (-0.202995) | 0.039245 / 0.043533 (-0.004288) | 0.250420 / 0.255139 (-0.004719) | 0.273642 / 0.283200 (-0.009557) | 0.019340 / 0.141683 (-0.122342) | 1.176734 / 1.452155 (-0.275421) | 1.250643 / 1.492716 (-0.242074) |

### 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) | 0.181210 / 0.018006 (0.163204) | 1.070750 / 0.000490 (1.070261) | 0.000315 / 0.000200 (0.000115) | 0.000045 / 0.000054 (-0.000009) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.022905 / 0.037411 (-0.014507) | 0.064549 / 0.014526 (0.050023) | 0.077113 / 0.176557 (-0.099443) | 0.131976 / 0.737135 (-0.605159) | 0.081266 / 0.296338 (-0.215072) |

### 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) | 0.291363 / 0.215209 (0.076154) | 2.851691 / 2.077655 (0.774036) | 1.592815 / 1.504120 (0.088695) | 1.494550 / 1.541195 (-0.046645) | 1.516464 / 1.468490 (0.047974) | 0.583244 / 4.584777 (-4.001532) | 2.504907 / 3.745712 (-1.240805) | 3.183490 / 5.269862 (-2.086371) | 1.932854 / 4.565676 (-2.632823) | 0.067564 / 0.424275 (-0.356711) | 0.006587 / 0.007607 (-0.001020) | 0.346368 / 0.226044 (0.120324) | 3.428256 / 2.268929 (1.159327) | 1.994176 / 55.444624 (-53.450448) | 1.688116 / 6.876477 (-5.188360) | 1.767653 / 2.142072 (-0.374420) | 0.673867 / 4.805227 (-4.131360) | 0.125582 / 6.500664 (-6.375082) | 0.047198 / 0.075469 (-0.028271) |

### 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) | 1.002895 / 1.841788 (-0.838893) | 16.332893 / 8.074308 (8.258585) | 10.781993 / 10.191392 (0.590601) | 0.153919 / 0.680424 (-0.526505) | 0.015528 / 0.534201 (-0.518673) | 0.306182 / 0.579283 (-0.273101) | 0.296380 / 0.434364 (-0.137984) | 0.341432 / 0.540337 (-0.198905) | 0.455900 / 1.386936 (-0.931036) |

</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) | 0.006442 / 0.011353 (-0.004911) | 0.004433 / 0.011008 (-0.006576) | 0.053327 / 0.038508 (0.014819) | 0.035966 / 0.023109 (0.012856) | 0.280913 / 0.275898 (0.005015) | 0.308419 / 0.323480 (-0.015061) | 0.005842 / 0.007986 (-0.002144) | 0.003789 / 0.004328 (-0.000539) | 0.053983 / 0.004250 (0.049732) | 0.069052 / 0.037052 (0.032000) | 0.299225 / 0.258489 (0.040736) | 0.336470 / 0.293841 (0.042629) | 0.068170 / 0.128546 (-0.060377) | 0.012259 / 0.075646 (-0.063388) | 0.064166 / 0.419271 (-0.355106) | 0.037291 / 0.043533 (-0.006241) | 0.281318 / 0.255139 (0.026179) | 0.297093 / 0.283200 (0.013893) | 0.021358 / 0.141683 (-0.120324) | 1.189584 / 1.452155 (-0.262571) | 1.256985 / 1.492716 (-0.235731) |

### 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) | 0.216726 / 0.018006 (0.198720) | 2.496957 / 0.000490 (2.496467) | 0.000336 / 0.000200 (0.000136) | 0.000070 / 0.000054 (0.000016) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.026604 / 0.037411 (-0.010807) | 0.080398 / 0.014526 (0.065873) | 0.094475 / 0.176557 (-0.082082) | 0.136263 / 0.737135 (-0.600873) | 0.097898 / 0.296338 (-0.198440) |

### 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) | 0.295171 / 0.215209 (0.079962) | 2.947530 / 2.077655 (0.869875) | 1.607531 / 1.504120 (0.103411) | 1.485045 / 1.541195 (-0.056150) | 1.524899 / 1.468490 (0.056409) | 0.572934 / 4.584777 (-4.011843) | 2.544320 / 3.745712 (-1.201393) | 3.292630 / 5.269862 (-1.977232) | 1.927138 / 4.565676 (-2.638539) | 0.068560 / 0.424275 (-0.355715) | 0.005982 / 0.007607 (-0.001625) | 0.345833 / 0.226044 (0.119789) | 3.424253 / 2.268929 (1.155324) | 2.195017 / 55.444624 (-53.249608) | 1.712037 / 6.876477 (-5.164440) | 1.763899 / 2.142072 (-0.378174) | 0.653776 / 4.805227 (-4.151451) | 0.123056 / 6.500664 (-6.377609) | 0.044572 / 0.075469 (-0.030897) |

### 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) | 1.033400 / 1.841788 (-0.808388) | 15.409887 / 8.074308 (7.335579) | 11.220990 / 10.191392 (1.029597) | 0.153603 / 0.680424 (-0.526821) | 0.016866 / 0.534201 (-0.517335) | 0.311945 / 0.579283 (-0.267338) | 0.307048 / 0.434364 (-0.127316) | 0.350422 / 0.540337 (-0.189915) | 0.447308 / 1.386936 (-0.939628) |

</details>
</details>

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