Do not call update-fullfiletimelist if there's no modules updated

Signed-off-by: Patrick Uiterwijk <patrick@puiterwijk.org>

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

@@ -103,6 +103,9 @@ def run_command(cmd):
 
 
 def update_fullfilelist(modules):
+    if not modules:
+        logger.info('No modulese to update')
+        return
     cmd = ['/usr/local/bin/update-fullfiletimelist', '-l',
            '/tmp/update-fullfiletimelist.lock', '-t', '/pub']
     cmd.extend(modules)