NULLs sometimes rendering as "None" and sometimes as "∅ null" due to string-casting fallback

[tconbeer]

Discussed in #655

This is wrong:
https://github.com/tconbeer/textual-fastdatatable/blob/739a8eeccb8e6f9c01142bbf12cd658d1a0b0564/src/textual_fastdatatable/backend.py#L255

should be:

new_data = {k: [str(val) if val is not None else None for val in v] for k, v in data.items()}