ARTEMIS-4962 mitigate NPE in FilterImpl

Author: jbertram

artemis-server/src/main/java/org/apache/activemq/artemis/core/filter/impl/FilterImpl.java

@@ -163,14 +163,17 @@ private static Object getHeaderFieldValue(final Message msg, final SimpleString
             String amqpNativeID = msg.getStringProperty(NATIVE_MESSAGE_ID);
             if (amqpNativeID != null) {
                return SimpleString.of(amqpNativeID);
+            } else {
+               return null;
             }
-         }
-         // It's the stringified (hex) representation of a user id that can be used in a selector expression
-         String userID = msg.getUserID().toString();
-         if (userID.startsWith("ID:")) {
-            return SimpleString.of(userID);
          } else {
-            return SimpleString.of("ID:" + msg.getUserID());
+            // It's the stringified (hex) representation of a user id that can be used in a selector expression
+            String userID = msg.getUserID().toString();
+            if (userID.startsWith("ID:")) {
+               return SimpleString.of(userID);
+            } else {
+               return SimpleString.of("ID:" + msg.getUserID());
+            }
          }
       } else if (FilterConstants.ACTIVEMQ_PRIORITY.equals(fieldName)) {
          return (int) msg.getPriority();