marker class for indicating that a PG is unlikely to ever be filled.
The "null" version of a PG is available only via the static data member
WhateverPG.nullPG, where WhateverPG is the base PG interface.
If an Asset has an actual null as the value for a given PG, it is
interpreted as unspecified , and potentially available for
late-binding.
This is a simple class to support the perturbation
scenario of making a transportation link unavailable.
We need it because the perturbator plugin does not
support changing an object availability through the
role schedule (in xml).