[C11] Correct global atomic pointer initialization from an integer constant

This is a follow-up to 2e275e24 and
1395cde2 which corrects a missed case:
initializing an _Atomic(T *) from a null pointer constant in the form
of the integer literal 0.

Fixes https://github.com/llvm/llvm-project/issues/63550
Differential Revision: https://reviews.llvm.org/D154284