addTopic->subscribe - Adding a topic while the client is active always yields an exception

Please fill out the form below before submitting, thank you!

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

***

I am building a Spring application where I have to subscribe to topics during runtime. Using the Spring integration's `Mqttv5PahoMessageDrivenChannelAdapter` I am calling `addTopics()`, which subsequently ends up calling `subscribe()` on `MqttAsyncClient`. **The problem arises when adding a subscription when the client is already active.** Here is a trace of the chain of events:

```
Caused by: java.lang.IndexOutOfBoundsException: Index 0 out of bounds for length 0
at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:64) ~[na:na]
at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckIndex(Preconditions.java:70) ~[na:na]
at java.base/jdk.internal.util.Preconditions.checkIndex(Preconditions.java:266) ~[na:na]
at java.base/java.util.Objects.checkIndex(Objects.java:359) ~[na:na]
at java.base/java.util.ArrayList.get(ArrayList.java:427) ~[na:na]
at org.eclipse.paho.mqttv5.client.MqttAsyncClient.subscribe(MqttAsyncClient.java:1276) ~[org.eclipse.paho.mqttv5.client-1.2.5.jar:na]
at org.eclipse.paho.mqttv5.client.MqttAsyncClient.subscribe(MqttAsyncClient.java:1205) ~[org.eclipse.paho.mqttv5.client-1.2.5.jar:na]
at org.springframework.integration.mqtt.inbound.Mqttv5PahoMessageDrivenChannelAdapter.addTopic(Mqttv5PahoMessageDrivenChannelAdapter.java:257) ~[spring-integration-mqtt-6.0.2.jar:6.0.2]
at org.springframework.integration.mqtt.inbound.AbstractMqttMessageDrivenChannelAdapter.addTopics(AbstractMqttMessageDrivenChannelAdapter.java:301) ~[spring-integration-mqtt-6.0.2.jar:6.0.2]
```

It seems that the issue arises from the fact that subscribe is called with a freshly constructed `MqttProperties` -> `MqttAsyncClient.java:1205`. These properties are evaluated in `MqttAsyncClient.java:1276`:
```java
@Override
public IMqttToken subscribe(MqttSubscription[] subscriptions, Object userContext, MqttActionListener callback,
IMqttMessageListener messageListener, MqttProperties subscriptionProperties) throws MqttException {

int subId = subscriptionProperties.getSubscriptionIdentifiers().get(0); // <- Issue is here, properties are new so the ArrayList is not populated
```

This obviously throws an exception and prevents the client from adding any topics at runtime.