ref: remove nullability of default_identity

[asottile-sentry]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...entry/integrations/bitbucket_server/integration.py	60.00%	2 Missing ⚠️
src/sentry/integrations/gitlab/integration.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #89734   +/-   ##
=======================================
  Coverage   87.71%   87.71%           
=======================================
  Files       10186    10186           
  Lines      574668   574655   -13     
  Branches    22635    22635           
=======================================
- Hits       504052   504044    -8     
+ Misses      70188    70183    -5     
  Partials      428      428           

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ IntegrationError: Identity not found. /api/0/projects/{organization_id_or_slug}/{proj... View Issue

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