fix: user service strips none, so we need to use a 'real' username

[mesemus]

This fix can be removed when inveniosoftware/invenio-users-resources#207 gets merged.

[codecov]

Codecov Report

❌ Patch coverage is 11.11111% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.86%. Comparing base (f656e80) to head (aef70ae).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...oidc_einfra/services/components/aai_invitations.py	11.11%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
- Coverage   66.15%   65.86%   -0.30%     
==========================================
  Files          23       23              
  Lines        1445     1453       +8     
  Branches      185      185              
==========================================
+ Hits          956      957       +1     
- Misses        489      496       +7     

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