[PECOFF] Skip empty .drectve sections.

There are many object files in the standard library who have empty .drective
sections. Parsing the empty string is not wrong but a waste.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@198067 91177308-0d34-0410-b5e6-96231b3b80d8