fix: handle missing and plaeholder usernames gracefully

[lwasser]

closes #386

more soon...

[codecov]

Codecov Report

❌ Patch coverage is 89.65517% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.86%. Comparing base (ded9bd4) to head (7eb1936).

Files with missing lines	Patch %	Lines
src/pyosmeta/cli/update_review_teams.py	75.00%	1 Missing and 1 partial ⚠️
src/pyosmeta/models/base.py	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #387      +/-   ##
==========================================
- Coverage   79.31%   76.86%   -2.46%     
==========================================
  Files          13       14       +1     
  Lines         817      899      +82     
  Branches      106      119      +13     
==========================================
+ Hits          648      691      +43     
- Misses        155      192      +37     
- Partials       14       16       +2     

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