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.
Right now, the *only* restriction to construct a custom resource model is that the subject of all statements must be the same URI. It doesn't rename blank nodes. But I agree, there should be a better way (next commit).