Devices `value_type` shall not be `None`

[farmio]

Devices (Sensor, Number, ExposeSensor) shall not accept None for value_type. This would raise anyway.
An exception would be Notification as it uses a default value - but this may still be handled differently (using a default argument).

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Please make sure to update to the latest version of xknx (or Home Assistant) and check if that solves the issue. Let us know if that works for you by adding a comment 👍 This issue has now been marked as stale and will be closed if no further activity occurs. Thank you for your contributions.