Reject invalid configuration for the default MQTT server (#6066)

* Sanity check configuration for the default MQTT server

* Skip for MESHTASTIC_EXCLUDE_MQTT

---------

Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

src/mqtt/MQTT.h

@@ -61,6 +61,8 @@ class MQTT : private concurrency::OSThread
 
     bool isUsingDefaultServer() { return isConfiguredForDefaultServer; }
 
+    static bool isValidConfig(const meshtastic_ModuleConfig_MQTTConfig &config);
+
   protected:
     struct QueueEntry {
         std::string topic;