Handle empty default arguments correctly

Summary: If an empty IndexedString is used as default argument, the default argument is ignored. This allows language plugins to add default arguments even if later arguments do not have defaults.

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: zhigalin, kfunk, brauch, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D7031