[stable8] fix(NcAvatar): redirect to guest avatar if avatar doesnt exist

[backportbot]

Backport of PR #8440

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.52%. Comparing base (58e08ac) to head (dbefabf).
⚠️ Report is 11 commits behind head on stable8.

Additional details and impacted files

@@           Coverage Diff            @@
##           stable8    #8445   +/-   ##
========================================
  Coverage    46.52%   46.52%           
========================================
  Files          194      194           
  Lines         4890     4890           
  Branches      1262     1262           
========================================
  Hits          2275     2275           
  Misses        2527     2527           
  Partials        88       88           

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