new-updates-sync: allow for just syncing ostrees

Previously it was not able to sync just an ostree and
not a repo (i.e. an empty to: [], like we have right now
for f28).

roles/bodhi2/backend/files/new-updates-sync

@@ -311,6 +311,9 @@ def sync_single_release(release):
 
     for repo in RELEASES[release]['repos']:
         target = determine_last_link(release, repo)
+
+        # if "to" is not empty then sync repo
+        if RELEASES[release]['repos'][repo]['to']:
             curstatefile = os.path.join(
                 RELEASES[release]['repos'][repo]['to'][0]['dest'], 'state')
             curstate = None