Avoid throwing exception due to deleted change

When a refupdated event is fired due to the deletion of a change (ie.
deleting a draft), the plugin attempts to post a stream event related
to the change, but an unhelpful exception is thrown because the change
no longer exists. Replace exception with a null check.

Change-Id: I7819d3b249c456f9d2749b808a9e67fd4759e3e8

src/main/java/com/googlesource/gerrit/plugins/replication/PushResultProcessing.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/PushResultProcessing.java b/src/main/java/com/googlesource/gerrit/plugins/replication/PushResultProcessing.java
index bc503ac..fd4cebf 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/PushResultProcessing.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/PushResultProcessing.java
@@ -179,7 +179,10 @@
         try {
           ReviewDb db = schema.open();
           try {
-            hooks.postEvent(retrieveChange(ref, db), event, db);
+            Change change = retrieveChange(ref, db);
+            if (change != null) {
+              hooks.postEvent(change, event, db);
+            }
           } finally {
             db.close();
           }
@@ -196,9 +199,6 @@
         throws OrmException, NoSuchChangeException {
       PatchSet.Id id = PatchSet.Id.fromRef(ref);
       Change change = db.changes().get(id.getParentKey());
-      if (change == null) {
-        throw new NoSuchChangeException(id.getParentKey());
-      }
       return change;
     }
   }