Handle missing n5 blocks

[tischi]

@ctrueden I know @tpietzsch is still on holidays, but is there any chance that also this PR could be merged? Maybe @axtimwalde could have a look whether this is acceptable. It is required to properly open N5 files, because the specification allows for missing blocks.

[axtimwalde]

Thanks!

[imagesc-bot]

This pull request has been mentioned on Image.sc Forum. There might be relevant details there:

https://forum.image.sc/t/viewing-live-acquisitions-with-bigdataviewer/55879/8