Make GetPossibleUserByID can handle deleted user (#37430)

Make sure deleted user won't cause 500 error, simplify the caller's code

services/convert/convert.go

@@ -831,7 +831,7 @@ func ToOAuth2Application(app *auth.OAuth2Application) *api.OAuth2Application {
 
 // ToLFSLock convert a LFSLock to api.LFSLock
 func ToLFSLock(ctx context.Context, l *git_model.LFSLock) *api.LFSLock {
-	u, err := user_model.GetPossibleUserByID(ctx, l.OwnerID)
+	_, u, err := user_model.GetPossibleUserByID(ctx, l.OwnerID)
 	if err != nil {
 		return nil
 	}