Fix avatar icon missing when Gravatar is disabled

What does this MR do?

Fixes the default avatar (no_avatar.png) in the commit list and issue assignment when Gravatar is disabled.

Previously the icon was completely missing due to wrong path / no path in the img tag.

Why was this MR needed?

Fix for #38715 (closed) and #34733 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

#38715 (closed) and #34733 (closed)