Backport #31187Resolves#31167.
https://github.com/go-gitea/gitea/pull/30885 changed the behavior of
`repo.AvatarLink()` where it can now take the empty string and append it
to the app data URL. This does not point to a valid avatar image URL,
and, as the issue mentions, previous Gitea versions returned the empty
string.
Co-authored-by: Kemal Zebari <60799661+kemzeb@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>