bot_update: Don't patch if gerrit_repo or gerrit_ref are empty/None.

We should apply the patch only when gerrit_repo and gerrit_ref are passed,
instead of using only the --apply-patch-on-gclient flag.

R=agable@chromium.org

Bug: 643346
Change-Id: I28547aed9daeefa2657a108e0bacbfd4fc9fa07e
Reviewed-on: https://chromium-review.googlesource.com/989667
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -354,7 +354,7 @@ def gclient_sync(
       revision = 'origin/master'
     args.extend(['--revision', '%s@%s' % (name, revision)])
 
-  if apply_patch_on_gclient:
+  if apply_patch_on_gclient and gerrit_repo and gerrit_ref:
     # TODO(ehmaldonado): Merge gerrit_repo and gerrit_ref into a patch-ref flag
     # and add support for passing multiple patch refs.
     args.extend(['--patch-ref', gerrit_repo + '@' + gerrit_ref])