Charger commands validation error handling improvement

Summary
Improved validation and error responses when sending commands to chargers that are missing or not in a valid state. Previously, these cases could lead to internal ArgumentNullExceptions and a generic OperationFailedForUnknownReasons error.

New behavior
Instead of returning OperationFailedForUnknownReasons, the endpoint now responds more explicitly:

• If the charger does not exist (charger == null):
  Returns UnknownObject (aligned with other public charger endpoints).

• If the charger is deactivated or its deviceId is null/empty:
  Returns UnknownDeviceId.

Impact

• No changes to successful calls; valid chargers behave as before.
• For invalid or deactivated chargers, clients now receive clearer, more specific error codes instead of a generic failure reason.