[demangler] Fix demangling a template argument which happens to be a null pointer

As seen in https://github.com/llvm/llvm-project/issues/51854
llvm-cxxfilt was having trouble demangling the case "_Z1fIDnLDn0EEvv".
We handled the "LDNE" case and "LPi0E" but not "LDn0E". This change adds
that handling.

Differential Revision: https://reviews.llvm.org/D124010