[clangd] Make version in PublishDiagnosticsParams optional

Summary: We were serializing it no matter what, which was against the spec

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79692