[Sema] haveSameParameterTypes - replace repeated isNull() test with assertions

As reported on https://pvs-studio.com/en/blog/posts/cpp/0771/ (Snippet 2) - (and mentioned on rGdc4259d5a38409) we are repeating the T1.isNull() check instead of checking T2.isNull() as well, and at this point neither should be null - so we're better off with an assertion.

Differential Revision: https://reviews.llvm.org/D107347