Fix matching events without approvals.

Some events lack approvals (just a text review, for instance).
Make sure that we set approvals to [] rather than None in that
case to avoid problems in the event matching code which assumes
it is always a list.

Change-Id: I2cd95e3dd0a6254ed3ddaee320cf3316487cea32
diff --git a/zuul/trigger/gerrit.py b/zuul/trigger/gerrit.py
index 2f8d764..56aa1ee 100644
--- a/zuul/trigger/gerrit.py
+++ b/zuul/trigger/gerrit.py
@@ -41,7 +41,7 @@
             if patchset:
                 event.patch_number = patchset.get('number')
                 event.refspec = patchset.get('ref')
-            event.approvals = data.get('approvals')
+            event.approvals = data.get('approvals', [])
         refupdate = data.get('refUpdate')
         if refupdate:
             event.project_name = refupdate.get('project')