zarr-python: Incorrect default fill value causes byte arrays to become numeric when `write_empty_chunks=False`

Minimal example:

import zarr
a = zarr.create((1,), dtype=bytes)
a[0] = b''
assert a[0] == b''

Traceback (most recent call last):
  File "example.py", line 5, in <module>
    assert a[0] == b''
AssertionError

The value of a[0] is actually 0, when it should be b''.

Found by one of our users at https://github.com/tskit-dev/tsinfer/issues/628 this bug was introduced in the latest release (v2.11.0) in this commit: https://github.com/zarr-developers/zarr-python/commit/f461eb78fbb88187582cd9123d6ec7622d9abd26 when the default for write_empty_chunks was changed to False. The default fill_value for arrays created via zarr.creation.create is 0, so when an empty, unwritten chunk is re-created the previous value b'' becomes 0. I assume this fill_value should be None.