TrkVertexFitterUtils: Do not try to retrieve extrapolator for simple unit test.
This is another MR trying to deal with problems exposed by !47792 (merged).
In this case, it's not really a problem, because the unit test didn't need an extrapolator and so it didn't matter it wasn't configured. Nevertheless, once !47792 (merged) is in, this will fail and so it needs fixing.
And I'd argue this is cleaner anyway (and makes the test a tiny bit faster)