Don't require 'commit' attribute in merge event

Problem occurs when merge events for periodic pipeline
are processed: first, a trivial mistake in type checking
introduced in review 243493,
second, merge events generated for NullChange's are repo updates,
and don't provide 'commit' attribute.
I don't see the necessity to enforce 'build_set.commit' to be set
(which is given to job env ZUUL_COMMIT if 'ref' or 'refspec' are set,
but that's not the case for NullChange)

Change-Id: Ib5da4ba987898d37d8e5082f4b8e2a5c31910323
diff --git a/zuul/scheduler.py b/zuul/scheduler.py
index 05eb6fe..9f370b9 100644
--- a/zuul/scheduler.py
+++ b/zuul/scheduler.py
@@ -1577,9 +1577,9 @@
         if event.merged:
             build_set.commit = event.commit
         elif event.updated:
-            if not isinstance(item, NullChange):
+            if not isinstance(item.change, NullChange):
                 build_set.commit = item.change.newrev
-        if not build_set.commit:
+        if not build_set.commit and not isinstance(item.change, NullChange):
             self.log.info("Unable to merge change %s" % item.change)
             self.pipeline.setUnableToMerge(item)