Revert "Use empty() instead of size() == 0."

This reverts commit r194551 because it broke the buildbot.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@194552 91177308-0d34-0410-b5e6-96231b3b80d8
Re-submit r194551: Use empty() instead of size() == 0.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@194556 91177308-0d34-0410-b5e6-96231b3b80d8