Make internal Zarr-checksumming tree types support empty Zarrs

[jwodder]

@yarikoptic Please release this so I can use it in backups2datalad.

[codecov]

Codecov Report

Base: 87.92% // Head: 87.94% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (f298c46) compared to base (4f50102).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1138      +/-   ##
==========================================
+ Coverage   87.92%   87.94%   +0.01%     
==========================================
  Files          73       73              
  Lines        8572     8584      +12     
==========================================
+ Hits         7537     7549      +12     
  Misses       1035     1035              

Flag	Coverage Δ
unittests	87.94% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
dandi/cli/tests/test_digest.py	100.00% <100.00%> (ø)
dandi/support/digests.py	100.00% <100.00%> (ø)
dandi/support/tests/test_digests.py	100.00% <100.00%> (ø)

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[yarikoptic]

is an empty directory is a legit zarr and we will get those to archive? sorry if I am not seeing the connection (there is no reference) to other ongoing discussions.

[jwodder]

@yarikoptic While the CLI will refuse to upload an empty Zarr, it's technically possible to create one via the API (including unintentionally if an upload is cancelled during the first batch), and the backups2datalad tests include backing up an empty Zarr.

[yarikoptic]

gotcha, invalid but still possible

[github-actions]

🚀 PR was released in 0.46.5 🚀