fix(NcAvatar): do no request avatar image if icon slot provided

[Antreesy]

☑️ Resolves

• Fix component appearance
  • 'fake' preloaded user status can be provided, but it should not be limited to 'users' only, (e.g. there can be federated users and guests having it in the future)
  • avatar should not be requested by url from server, if <slot #icon> is provided by app and therefore won't be ever shown

🖼️ Screenshots

🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable
• 2️⃣ Backport to stable8 for maintained Vue 2 version or not applicable

[ShGKme]

fix(NcAvatar): do no request avatar image if icon slot provided

Most of the changes are not related to the PR title (release note) 👀

[ShGKme]

Please, the next time split into 2 PRs for 2 independent changes, or do not forget to manually adjust the changelog

[Antreesy]

/backport to stable8

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.14%. Comparing base (0074857) to head (121badd).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/components/NcAvatar/NcAvatar.vue	75.00%	1 Missing ⚠️
src/mixins/userStatus.js	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7891      +/-   ##
==========================================
+ Coverage   52.11%   52.14%   +0.03%     
==========================================
  Files         100      100              
  Lines        3170     3170              
  Branches      872      871       -1     
==========================================
+ Hits         1652     1653       +1     
  Misses       1271     1271              
+ Partials      247      246       -1     

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