[clang-format] Fix regression when getStyle() called with empty filename

Summary:
D43522 caused an assertion failure when getStyle() was called with
an empty filename:

P8065

This adds a test to reproduce the failure and fixes the issue by
ensuring we never pass an empty filename to
Environment::CreateVirtualEnvironment().

Test Plan: New test added. Ran test with:
  % make -j12 FormatTests && ./tools/clang/unittests/Format/FormatTests
  Before diff, test failed with P8065. Now, test passes.

Reviewers: vsapsai, jolesiak, krasimir

Reviewed By: vsapsai

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D43590

llvm-svn: 325722