Test for empty .clang-format file.

Summary: Test clang-format's handling of empty .clang-format files.

Reviewers: djasper

Reviewed By: djasper

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D1562

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@189757 91177308-0d34-0410-b5e6-96231b3b80d8