[clangd] Add a testcase for empty preamble.

Summary: This is a patch of add a testcase for https://reviews.llvm.org/D50628.

Reviewers: ilya-biryukov

Subscribers: javed.absar, ioeric, MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D50627

llvm-svn: 340035