Scott Marlow just reading this too, and I agree with Steve: the specification sections you quote are not stating that this method should return null. So please clarify why you think this needs to be done.

To break it down:

After the close method has been invoked, all methods on the EntityManager instance and any Query, TypedQuery, and StoredProcedureQuery objects obtained from it will throw the IllegalStateException except for getProperties, getTransaction, and isOpen (which will return false).

This section is stating that several methods except getTransaction, getProperties and isOpen will throw an exception.

Additionally it states that the method isOpen will return false.

Most importantly it does not specify what the method getTransaction will return.