[Sema] Don't treat a non-null template argument as if it were null.

The way this code checks whether a pointer is null is wrong for other
reasons; it doesn't actually check whether a null pointer constant is a
"constant" in the C++ standard sense.  But this fix at least makes sure
we don't treat a non-null pointer as if it were null.

Fixes https://github.com/llvm/llvm-project/issues/57883

Differential Revision: https://reviews.llvm.org/D134928