Purpose:Indicates an object that should not be returned from query execution.

When conforming if checkEarly return finds a matching object by exact primary key, but that object is deleted, want to return null from query execution.

However if null is returned from checkEarly return that will indicate that no object was found and to go to the database. Hence returning null is not enough, something else needed to be returned, indicating not only that checkEarlyReturn had failed but query execution should not proceed.

Can be used in other instances where returning null is ambiguous.

Implements singleton pattern