fix: implicit fill value initialisation

[LDeakin]

Fixes omissions in #2274

• initialise empty chunks to the default fill value during writing
• add default fill value for datetime, timedelta, structured, and other (void* fixed size) data types
• add test for "other" data type

Fixes the below:

array = zarr.create_array(
    store=zarr.storage.MemoryStore(),
    dtype=np.uint8,
    shape=(4,),
    chunks=(2,),
    fill_value=None,
    zarr_format=2,
)
array[:3] = [1, 2, 3]
print(array[:]) # [1, 2, 3, undefined]

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.rst
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[LDeakin]

This could use some keen eyes, I've never used these dtypes.

[d-v-b]

I'm also unfamiliar with those dtypes. perhaps you could find examples from the tests on the v2 branch?