Fix size_str(0) returning "Unknown size" instead of "0 bytes"

[Sreekant13]

size_str uses if not size_in_bytes: to return "Unknown size", which also catches 0. But 0 is a valid size (0 bytes), and the docstring states that only None means "Unknown size". So a 0-byte size (e.g. an empty split) currently renders as "Unknown size":

>>> from datasets.utils.py_utils import size_str
>>> size_str(0)
'Unknown size'   # expected '0 bytes'

This checks size_in_bytes is None instead. Added a regression test covering None, 0, and non-zero sizes.