Include all-zero buckets in volume annotation downloads

[fm3]

Fixed a bug where all-zero chunks/buckets were omitted when downloading volume annotation even in case of a fallback segmentation layer, where ther zeroed-bucket information is actually needed. Otherwise, it can’t be distinguished if the area should be black or should render the fallback layer data again.

Only “reverted” buckets (marked by a single byte with value zero) are still omitted even in case of fallback segmentation layers.

URL of deployed dev instance (used for testing):

• https://downloadvolumezeroedbuckets.webknossos.xyz

Steps to test:

• Create volume annotation on a dataset with an existing segmentation layer (e.g. l4_sample)
• With a large brush size erase a volume comprising multiple buckets (i.e in at least 32 sections)
• Download that volume annotation, reupload
• The transparent hole should still be there, and not look like this (old behavior)

Issues:

• fixes Volume Annotation Download: keep all-zero chunks to support merging with fallback layer #7532

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• Updated changelog