[PECOFF] Do not emit a section if it's empty.

The PE32+ loader does not seem to like an executable with an empty section, so
we should not emit a section if it's empty.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@200514 91177308-0d34-0410-b5e6-96231b3b80d8