fix(home-assistant): only end connection if connected

If the connection could not be established during boot mqttClient would
not have been set - and calling end would throw an error.

src/integrations/home-assistant/home-assistant.service.ts

@@ -87,7 +87,7 @@ export class HomeAssistantService
         );
       }
     });
-    return this.mqttClient.end();
+    return this.mqttClient?.end();
   }
 
   /**