[clangd] Fix hover crashing on null types

Summary: Fixes https://github.com/clangd/clangd/issues/225

Reviewers: sammccall, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71403