Fix NPE for recevied messages with missing public key

smack-openpgp/src/main/java/org/jivesoftware/smackx/ox/OpenPgpManager.java

@@ -599,6 +599,10 @@ public final class OpenPgpManager extends Manager {
                     return;
                 }
 
+                if (contentElement == null) {
+                    return;
+                }
+
                 else {
                     throw new AssertionError("Invalid element received: " + contentElement.getClass().getName());
                 }