ReplicationQueue: Check nulls in firePendingEvents

After a sudden reboot (for unknown reason) Gerrit at startup couldn't
load because of NullPointerException. There is a possibility that
stored event was null at that point. Extra logging added to handle null
events.

Change-Id: I72f34d8def6e0246196cd865f33f6e795b21664b

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

diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationQueue.java b/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationQueue.java
index c79363b..da5a4d2 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationQueue.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/ReplicationQueue.java
@@ -170,8 +170,12 @@
 
   private void firePendingEvents() {
     for (EventsStorage.ReplicateRefUpdate e : eventsStorage.list()) {
-      repLog.info("Firing pending event {}", e);
-      onGitReferenceUpdated(e.project, e.ref);
+      if (e == null) {
+        repLog.warn("Encountered null replication event in EventsStorage");
+      } else {
+        repLog.info("Firing pending event {}", e);
+        onGitReferenceUpdated(e.project, e.ref);
+      }
     }
   }