Avoid NPE if broker yet not started (#743)

Update Server's listConnectedClients to check that the server instance is initialized, to avoid NPE. If not throws an state exception.

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Co-authored-by: Andrea Selva <selva.andre@gmail.com>

broker/src/main/java/io/moquette/broker/Server.java

@@ -634,8 +634,13 @@ public void removeInterceptHandler(InterceptHandler interceptHandler) {
 
     /**
      * Return a list of descriptors of connected clients.
+     * Returns null if the broker is not started.
      * */
     public Collection<ClientDescriptor> listConnectedClients() {
+        if (!initialized) {
+            LOG.error("Moquette is not started, MQTT clients listing unavailable");
+            throw new IllegalStateException("Can't get clients list from a Server that is not yet started");
+        }
         return sessions.listConnectedClients();
     }
 }