Upon External Serialization, do a nulll check on modificationDate. It is...

Upon External Serialization, do a nulll check on modificationDate. It is possible that the modificationDate is null, if the room was just created through REST API. That results in an NPE,
partial room creation, and failure in communicating the room updated event via hazelcast.

If modification date is null, send the creationDate value instead.
see https://discourse.igniterealtime.org/t/openfire-hazelcast-clustering-rest-api-room-creation-npe-when-no-modificaitondate-sent/80619/2

src/java/org/jivesoftware/openfire/muc/spi/LocalMUCRoom.java

@@ -2723,7 +2723,12 @@ public class LocalMUCRoom implements MUCRoom, GroupEventListener {
         ExternalizableUtil.getInstance().writeSafeUTF(out, subject);
         ExternalizableUtil.getInstance().writeLong(out, roomID);
         ExternalizableUtil.getInstance().writeLong(out, creationDate.getTime());
+        if (modificationDate!=null) {
             ExternalizableUtil.getInstance().writeLong(out, modificationDate.getTime());
+        }
+        else {
+            ExternalizableUtil.getInstance().writeLong(out, creationDate.getTime());
+        }
         ExternalizableUtil.getInstance().writeBoolean(out, emptyDate != null);
         if (emptyDate != null) {
             ExternalizableUtil.getInstance().writeLong(out, emptyDate.getTime());