bundles/systemd: fix edgecase in check_timesyncd_sync

bundles/systemd/files/check_timesyncd_sync

@@ -29,6 +29,11 @@ try:
         print('NTP service is active, but could not get offset. Something is wrong.')
         exit(1)
 
+    if out in ('+0', '-0', '0'):
+        # if the offset is exactly zero, timedatectl only shows '0', thus
+        # breaking the logic below.
+        out = '0ms'
+
     if out.startswith('+'):
         out = out[1:]