Fixing error return to be consistent with tuple return at the end of the method

In passing I managed to produce a situation where I would get an error when this check fails. When this gets called on line 391:

```python
391            # Multi-part file uploads.
392            if files:
393                (body, content_type) = self._encode_files(files, data)
```

Which would result in:
```TypeError: 'NoneType' object is not iterable```

This patch fixes that return.