Null checking setResponseTopic

Signed-off-by: James Sutton <james.sutton@uk.ibm.com>

Author: jpwsutton

org.eclipse.paho.mqttv5.common/src/main/java/org/eclipse/paho/mqttv5/common/packet/MqttProperties.java

@@ -1226,7 +1226,9 @@ public String getResponseTopic() {
 	 *            The Response Topic.
 	 */
 	public void setResponseTopic(String responseTopic) {
-		MqttTopicValidator.validate(responseTopic, false);
+		if(responseTopic != null) {
+			MqttTopicValidator.validate(responseTopic, false);
+		}
 		this.responseTopic = responseTopic;
 	}