do not allow schemas with $id of "" to be persistent

[karenetheridge]

It would be easy enough to delete the entry for "" in the resource index at the end of the evaluate() sub.
I can't forsee any strange side effects from this, as no other schema would be able to $ref to this schema anyway (if there is another, it would have an $id, and therefore any $ref it uses would be resolved against its own base and not "").