Improve qb validation

fixes some bugs in the tests, also fixes https://github.com/doctrine/phpcr-odm/issues/541

note ``Doctrine\Tests\ODM\PHPCR\Query\Builder\BuilderConverterPhpcrTest::testOrderBy`` is failing .. but honestly I don't really think such a test makes sense. It is way too complex to test as it requires so much mocking. IMHO we are better off testing this via the various existing functional test. So I vote for removing that test.

/cc @dantleech 

Not all nodes have the `getField` method, this is why the test failed.