Allow using domain specific models in testNullRelativeUri

Currently, *testNullRelativeUri* depends on a hard-coded model for testing null relative URIs. As described in https://github.com/w3c/ldp-testsuite/pull/35, this test always fails when the server is domain-specific.

**Proposal**
Use the *postContent* model (Bug or custom) and verify if the intersection between the requestModel and the responseModel is not empty.
Doesn't this have the drawback that ANY statement's subject node is overwritten?  So the user would have to be careful to construct this model knowing this.  Is there a better way?