MOF14 — Disallow null instance values

It can be argued that there is no need to allow null as a valid value
for a Class instance. The possibility of null means that mapped APIs
need to be more complicated; e.g. the IDL "unset_*" operations. This
issue proposes that 'null' should be disallowed, making Class instances
consistent with DataType instances. The IDL mapping could also be
simplified.