controller should set agv state unkonwn when missing state msg about opentcs-commadapter-vda5050 HOT 1 CLOSED

@vincentknw The VDA5050 specification describes a clear indication of whether or not a vehicle is available in the form of the connection topic and the vehicle's last will:

• When the vehicle disconnects gracefully, it is supposed to publish OFFLINE as its connectionState.
• For cases in which the vehicle's connection to the MQTT broker is interrupted unexpectedly, the vehicle is supposed to register a last will that publishes CONNECTIONBROKEN as its connectionState.

As long as the vehicle adheres to the specification, checking for incoming state messages is neither required by the specification nor necessary.

from opentcs-commadapter-vda5050.