Volume Annotation Download: keep all-zero chunks to support merging with fallback layer

[fm3]

Volume annotation downloads currently skip all-zero chunks. For annotations with fallback segmentations layers, this means the information whether a chunk was drawn over using all black, or whether it is untouched, is lost.

Original report: scalableminds/webknossos-libs#974

I think just removing the !isAllZero(data) in Zarr3BucketStreamSink and WKWBucketStreamSink might not be enough, I have vague memories of a separate mechanism to indicate all-overwritten buckets, possibly also something with version restores. We should look into this in more detail.

Consider only including all-zero chunks in case of a fallback layer being present