allow `null` attributes

[d-v-b]

We currently error if the attributes field of zarr.json is missing. We should handle this instead of erroring.

reported by @joshmoore

Edit: data was originally written by tensorstore

[jhamman]

👍 TIL that attributes is indeed optional in the v3 metadata doc.

[d-v-b]

yeah, my preference would be for it not to be optional, since an empty object sufficiently conveys "no attributes" 🤷 I will get a fix out for this soon.