Fix the issue addressed in r243996: avoid calling std::equals on nullptr

Fix the issue addressed in r243996: avoid calling std::equals on nullptr
because MSVC's STL implementation can trip a debug assert on that.

There is still a discussion ongoing about r243996, so let's just apply
a minimal fix for 3.7.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_37@245233 91177308-0d34-0410-b5e6-96231b3b80d8