[clangd] Return empty results on spurious completion triggers

Summary:
We currently return an error, this causes `coc.nvim` and VSCode to
show an error message in the logs.

Returning empty list of completions allows to avod the error message
without altering other user-visible behavior.

Reviewers: sammccall

Reviewed By: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62999

llvm-svn: 362811