Allow for null baseline_sha

[dberenbaum]

Needed for iterative/dvclive#646 to work without a baseline revision (since this would be a pain to provide in a no-git scenario).

@daavoo I will likely need your help to make similar changes on the Studio side to accept requests with missing baseline_sha.

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (c85eda7) 100.00% compared to head (cd8507f) 100.00%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #72   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            1         1           
  Lines          154       154           
  Branches         6         6           
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  Hits           154       154           

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[daavoo]

Can we use just a 40-character placeholder string like "z" * 40?

[dberenbaum]

Can we use just a 40-character placeholder string like "z" * 40?

The schema checks for a valid sha here:

dvc-studio-client/src/dvc_studio_client/schema.py

Line 9 in 535c3aa

Sha = All(Lower, Match(r"^[0-9a-h]{40}$"), msg="expected a length 40 commit sha")

[shcheklein]

@dberenbaum is it still relevant?

[dberenbaum]

It depends on how we decide to go with issues like iterative/dvclive#638 and iterative/dvclive#237. If we decide to clone the repo on every node where dvclive runs, then this won't be needed, but if we decided to allow for some "no-git" scenarios, I think it might be needed. I can convert to draft until we decide.