Resolves#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.
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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>