fix topic alias Always empty

- [x] Bug exists Release Version 1.2.5 ( Master Branch)
- [ ] Bug exists in MQTTv3 Client on Snapshot Version 1.2.6-SNAPSHOT (Develop Branch)
- [x] Bug exists in MQTTv5 Client on Snapshot Version 1.2.6-SNAPSHOT (Develop Branch)

fix topic alias Always empty
paho.mqtt.java/org.eclipse.paho.mqttv5.client/src/main/java/org/eclipse/paho/mqttv5/client/internal/ClientState.java: line 1193

```java
// Is this alias being sent with a topic string?
if (send.getTopicName() != null && !"".equals(send.getTopicName())) {
// @TRACE 652=Setting Incoming New Topic Alias alias={0}, topicName={1}
log.fine(CLASS_NAME, methodName, "652", new Object[] {
Integer.valueOf(send.getProperties().getTopicAlias()), send.getTopicName() ,incomingTopicAliases});
incomingTopicAliases.put(send.getProperties().getTopicAlias(), send.getTopicName());
} else {
// No Topic String, so must be in incomingTopicAliases.
if (incomingTopicAliases.containsKey(incomingTopicAlias)) {
send.setTopicName(incomingTopicAliases.get(incomingTopicAlias));
} else {
// @TRACE 654=Unknown Topic Alias: Incoming Alias={1}
log.severe(CLASS_NAME, methodName, "654",
new Object[] {
Integer.valueOf(send.getProperties().getTopicAlias()) ,
incomingTopicAliases,
incomingTopicAlias,
incomingTopicAliases.containsKey(incomingTopicAlias),
incomingTopicAliases.get(incomingTopicAlias)
});
throw new MqttException(MqttClientException.REASON_CODE_UNKNOWN_TOPIC_ALIAS);
}
}
```