Return default avatar url when user id is zero rather than updating database (go-gitea#34094) (go-gitea#34095)

Backport go-gitea#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 <[email protected]>

Author: GiteaBot

models/user/avatar.go

@@ -61,7 +61,9 @@ func GenerateRandomAvatar(ctx context.Context, u *User) error {
 
 // AvatarLinkWithSize returns a link to the user's avatar with size. size <= 0 means default size
 func (u *User) AvatarLinkWithSize(ctx context.Context, size int) string {
-	if u.IsGhost() || u.IsGiteaActions() {
+	// ghost user was deleted, Gitea actions is a bot user, 0 means the user should be a virtual user
+	// which comes from git configure information
+	if u.IsGhost() || u.IsGiteaActions() || u.ID <= 0 {
 		return avatars.DefaultAvatarLink()
 	}