roundtripping bytes with specific _FillValue fails on Zarr 3.0.7

What happened?

Seen in CI. I have xfailed the Zarr test in #10255

xarray/tests/test_backends.py::TestZarrDictStore::test_roundtrip_bytes_with_fill_value[2]: AssertionError: Left and right Dataset objects are not identical
Differing data variables:
L   x        (t) object 24B b'ab' b'cdef' nan
R   x        (t) object 24B b'ab' b'cdef' b'X'
xarray/tests/test_backends.py::TestZarrDirectoryStore::test_roundtrip_bytes_with_fill_value[2]: AssertionError: Left and right Dataset objects are not identical
Differing data variables:
L   x        (t) object 24B b'ab' b'cdef' nan
R   x        (t) object 24B b'ab' b'cdef' b'X'
xarray/tests/test_backends.py::TestZarrWriteEmpty::test_roundtrip_bytes_with_fill_value[2]: AssertionError: Left and right Dataset objects are not identical
Differing data variables:
L   x        (t) object 24B b'ab' b'cdef' nan
R   x        (t) object 24B b'ab' b'cdef' b'X'

This is probably an upstream issue, https://github.com/zarr-developers/zarr-python/issues/2979 but we should remember to remove the xfail when upstream is fixed.

cc @ianhi

Start with the three named tests in xarray/tests/test_backends.py and check upstream zarr-developers/zarr-python issue 2979 for the fix status. When the upstream issue is fixed, remove the xfail from the affected Zarr cases and run the listed roundtrip tests to confirm bytes with a specific _FillValue are preserved.