Only get client_id when GUI presents option

[AStoker]

When setting up the ESP32 Sense from the "Discovered" integration, it doesn't ask for the client id. However, the code is trying to access that entered client id, resulting in an error like this

2025-12-02 08:23:15.303 ERROR (MainThread) [custom_components.sensecraft.config_flow] Unexpected exception
Traceback (most recent call last):
File "/config/custom_components/sensecraft/config_flow.py", line 258, in async_step_mqtt
client_id = user_input[CLIENT_ID]
~~~~~~~~~~^^^^^^^^^^^
KeyError: 'client_id'

Instead, move the client_id setting down into the has_mqtt_topic which is when we know we'll actually need the client id.