[Clangd] Changed ExtractVariable to only work on non empty selections

Summary:
- For now, we don't trigger in any case if it's an empty selection
- Fixed unittests

Reviewers: kadircet, sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64912

llvm-svn: 366451