Re-cast definition of unique_key in SnapshotConfig

[gshank]

Resolves #

Problem

Python caching of unique_key definition in SnapshotConfig causes deserialization errors because of wrong order.

Solution

Change definition from Optional[Union[str, List[str]]] to Union[str, List[str], None].

Checklist

• I have read the contributing guide and understand what's expected of me.
• I have run this code in development, and it appears to resolve the stated issue.
• This PR includes tests, or tests are not required or relevant for this PR.
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• This PR includes type annotations for new and modified functions.

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the contributing guide.

[aranke]

Thanks for finding and fixing this!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.86%. Comparing base (b414ef2) to head (1ffb3d4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11205      +/-   ##
==========================================
- Coverage   88.88%   88.86%   -0.02%     
==========================================
  Files         187      187              
  Lines       24104    24104              
==========================================
- Hits        21425    21421       -4     
- Misses       2679     2683       +4     

Flag	Coverage Δ
integration	86.17% <100.00%> (-0.02%)	⬇️
unit	62.40% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
Unit Tests	62.40% <100.00%> (ø)
Integration Tests	86.17% <100.00%> (-0.02%)	⬇️