Data type failure on `develop`

[TomNicholas]

There are failures on develop with the upstream tests:

fill_value_parsed = data_type.cast_scalar(fill_value)
                            ^^^^^^^^^^^^^^^^^^^^^
E       AttributeError: 'str' object has no attribute 'cast_scalar'

Seems like a zarr v3 data types refactor issue. I notice that in #618 the upstream tests CI didn't seem to actually run...

[TomNicholas]

Note that one of the things causing breakage is that fill_value=None is no longer valid to pass to ArrayV3Metadata in general. That's because we were previously relying on numpy silently turning that into an appropriate default fill value for the dtype (e.g. NaN for a float).