[PECOFF] Skip empty .drectve sections.
[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: f6089bf0e6284f307027cef4f64114ee9ebb0424@198067 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld