[clangd] Fix a crash when reading an empty index file.

Summary:
Unfortunately, yaml::Input::setCurrentDocument() and yaml::Input::nextDocument() are
internal APIs, the way we use them may cause a nullptr accessing when
processing an empty YAML file.

Reviewers: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D56442

llvm-svn: 350633