Fix: Handle null DataBlock from NonEmptyDataBlockSupplier

[StephanPreibisch]

Fixes NullPointerException when writing blocks that are entirely filled with default values. NonEmptyDataBlockSupplier returns null for such blocks as an optimization, but the codec chain didn't handle this properly.

Changes:

1. DefaultDatasetAccess.writeRegionRecursive(): Add null check before encoding blocks. Return null to signal that empty blocks should not be written.

2. RawBlockCodecs.encode(): Add defensive null check to prevent NPE when creating LazyReadData with null dataBlock.

This allows N5Utils.saveNonEmptyBlock() to work correctly with Zarr codecs and prevents crashes when writing multi-resolution pyramids with partial blocks containing only default values.

Second commit, Fix: Skip pva.put() for null blocks in writeRegion()

Adds the third and final fix to handle empty blocks from
NonEmptyDataBlockSupplier. After writeRegionRecursive() returns null
for empty blocks, writeRegion() must check for null before calling
pva.put(). Without this check, a NullPointerException occurs at
PositionValueAccess.put() line 113.

Note, this is for ZarrV2 using N5ZarrWriter.

There is an extra issue reading empty blocks from a sharded ZarrV3 dataset, I am right now using N5Utils.saveBlock instead of N5Utils.saveNonEmptyBlock when saving sharded data.

Please note, this code was suggested by Claude that it would fix the issue, I have not thought it through since I am not deeply inside the N5-API. Importantly, a test that reproduces the error is available here: JaneliaSciComp/multiview-reconstruction@4789200

[StephanPreibisch]

related to this pull request in n5-zarr: saalfeldlab/n5-zarr#85