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.
```log
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`=?"
...
```
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
25e409e025