Only use None for "no filters"

[d-v-b]

In v2, we allow filters to be an empty array. But the v2 spec states that filters should be an array with at least one element, or None. So we should fix our implementation, and no longer generate metadata with an empty array of filters. For backwards compatibility, we should read the empty array of filters, and probably emit a warning.

[leuchthelp]

#2841 (comment)

Linking the discussion for context. Can also show up when creating a format=3 zarr file, but I do not know if it might be intended for zarr format=3.

[d-v-b]

#2841 (comment)

Linking the discussion for context. Can also show up when creating a format=3 zarr file, but I do not know if it might be intended for zarr format=3.

in the case of zarr v3, "filters" is a shorthand name for a type of codec that transforms arrays into arrays, and we do want to represent 0 of them as an empty tuple, so I don't think there's a problem there.