Resolve "PUT /projects/:id : unable to update avatar alone"

Hope it's OK that I tagged you as an approver, @DouweM -- GitLab suggested you!

What does this MR do?

Fixes issue #48537 (closed), wherein a user cannot update only the project avatar via the API.

Are there points in the code the reviewer needs to double check?

Is the single test sufficient, or should this be possible for other roles, too?

Why was this MR needed?

Bug.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• Conform by the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #48537 (closed)