Detect and handle when MQTT URI hasn't been set

[vringar]

Currently if the user forgot to setup their config properly, matterHub will just read the empty string and crash:

I (1245) MQTT: This is my name:  and this is my key
I (1245) MQTT: [APP] Free memory: 130232 bytes
Guru Meditation Error: Core  0 panic'ed (LoadProhibited). Exception was unhandled.

IDK if we can use __static_assert or something else in the build to detect that this is unset and refuse to build.
Or we crash at runtime with a descriptive error message.