[clangd] Fix a crash for accessing a null template decl returned by findExplicitReferences.

Summary: Fixes https://github.com/clangd/clangd/issues/347.

Reviewers: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78626