unit: Add unregistering with payload length zero
unit: Add unregistering with payload length zero

This patch test unregister operation sending an invalid data. The
expected result is' KNOT_INVALID_DATA'

CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source