Zarr v3 spec mentions "fill_value": null in an example

[rouault]

https://github.com/zarr-developers/zarr-specs/blob/214ba9b6326aadcf406a69a1a7268a7055cea185/docs/v3/core/v3.0.rst?plain=1#L765 is "fill_value": null. My understanding is that this is not allowed in Zarr V3

[d-v-b]

That example introduces a custom datetime dtype, which could be nullable, in which case null would be a valid element from that dtype. So I don't think this goes against the spec, but this example should be clarified.