Return default avatar url when user id is zero rather than updating database (#34094)

[GiteaBot]

Backport #34094 by @lunny

When visit commit list, it would update the user avatar even if id = 0, which was unnecessary operations. This PR returned default avatar for the git only user avatar rendering who's user id is zero.

database duration=0.0005s db.sql="UPDATE `user` SET `avatar` = ?, `updated_unix` = ? WHERE `id`=?"
database duration=0.0007s db.sql="UPDATE `user` SET `avatar` = ?, `updated_unix` = ? WHERE `id`=?"
...