[clang] Check for nullptr when instantiating late attrs

This was already done in SemaTemplateInstantiateDecl.cpp, but not in
SemaTemplateInstantiate.cpp.

Anecdotally I've seen some clangd crashes where coredumps point to this
being a problem, but I cannot reproduce this so far.

Differential Revision: https://reviews.llvm.org/D94933