[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

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@366451 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed