NULLs sometimes rendering as "None" and sometimes as "&empty; null" due to string-casting fallback

### Discussed in https://github.com/tconbeer/harlequin/discussions/655

This is wrong:
https://github.com/tconbeer/textual-fastdatatable/blob/739a8eeccb8e6f9c01142bbf12cd658d1a0b0564/src/textual_fastdatatable/backend.py#L255

should be:
```py
new_data = {k: [str(val) if val is not None else None for val in v] for k, v in data.items()}
```