Replace -1 with GhostUserID (#27703)

models/user/avatar.go

@@ -58,8 +58,7 @@ 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.ID == -1 {
-		// ghost user
+	if u.IsGhost() {
 		return avatars.DefaultAvatarLink()
 	}