Rework MQTT config merging and adding defaults (#90529)

* Cleanup config merging and adding defaults

* Optimize and update tests

* Do not mix entry and yaml config

* Make sure hass.data is initilized

* remove check on get_mqtt_data

* Tweaks to MQTT client

* Remove None assigment mqtt client and fix mock

homeassistant/components/mqtt/models.py

@@ -288,8 +288,8 @@ class EntityTopicState:
 class MqttData:
     """Keep the MQTT entry data."""
 
-    client: MQTT | None = None
-    config: ConfigType | None = None
+    client: MQTT
+    config: ConfigType
     debug_info_entities: dict[str, EntityDebugInfo] = field(default_factory=dict)
     debug_info_triggers: dict[tuple[str, str], TriggerDebugInfo] = field(
         default_factory=dict