Fix check for falsey inputs in run_summarization
Author: JohnGiorgi
Pull request: yes

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In the PyTorch version of `run_summarization.py`, there is a check that excludes examples where the source document and target summary are both `None`. 

https://github.com/huggingface/transformers/blob/ccc089780415445768bcfd3ac4418cec20353484/examples/pytorch/summarization/run_summarization.py#L516-L520

I think this should be relaxed to check for _falsey_ inputs instead. I think this because some datasets, like MultiNews, contain examples with empty strings:

```python
from datasets import load_dataset

multi_news = load_dataset("multi_news", split="validation")
assert not multi_news[4850]["document"]
```

and these are not caught by the `is not None` check.

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