deprecate / remove support for creating v2 array metadata with `null` fill value

[d-v-b]

Indeed! To be honest, I think null fill values in Zarr V2 were a bit of a misstep anyway, mainly because undefined values in partially written chunks cannot be distinguished from real data. This could be a good opportunity for zarr-python to stop writing null fill values altogether in Zarr V2 metadata and just write the default fill value. What do you think?

Originally posted by @LDeakin in https://github.com/zarr-developers/zarr-python/issues/2819#issuecomment-2690206937

Following up on this idea, I think we should consider altering our user-facing array creation routines to disallow creating a null fill value in v2 array metadata. Instead, we should treat fill_value=None to mean "use the default fill value for the dtype".

Users who want to create a zarr v2 array with a fill value that is null should be directed to lower-level routines for this.