Skip triggers if there are no ports in the list

buildbot/master.cfg

@@ -476,18 +476,23 @@ def make_portwatcher_factory(triggerable):
         name='subports',
         description=['listing', 'subports']))
 
+    def has_subportlist(step):
+        return step.hasProperty('subportlist') and step.getProperty('subportlist')
+
     if 'mirror' in config['deploy']:
         portwatcher_factory.addStep(steps.Trigger(
             schedulerNames=['mirror'],
             set_properties={'subportlist': Property('subportlist'), 'triggered_by': make_build_url},
             waitForFinish=True,
-            updateSourceStamp=True))
+            updateSourceStamp=True,
+            doStepIf=has_subportlist))
 
     portwatcher_factory.addStep(TriggerWithPortlist(
         schedulerNames=[triggerable],
         set_properties={'triggered_by': make_build_url},
         waitForFinish=True,
-        updateSourceStamp=True))
+        updateSourceStamp=True,
+        doStepIf=has_subportlist))
 
     # make a logfile summarising the success/failure status for each port
     # (Current approach is not so useful as it is not incremental;