fix(types): Validate user_id in Vercel post_install

[JoshFerge]

Summary

Add validation to ensure user_id from extra dict is not None before querying User model. Log error and return early if missing.

Changes

• Extract user_id from extra dict
• Add None check with error logging
• Return early if user_id is missing

Test plan

• Existing tests pass
• Mypy type checking passes

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/sentry/integrations/vercel/integration.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #103021      +/-   ##
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- Coverage   80.75%    80.64%   -0.11%     
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  Files        9145      9140       -5     
  Lines      395020    392797    -2223     
  Branches    24968     24968              
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- Hits       319002    316780    -2222     
+ Misses      75617     75616       -1     
  Partials      401       401