Fix problem with include paths with clang language parser

Summary:
Do not add empty paths to include arguments for clang parser

This fixed an issue I had with a custom build project.

I don't yet understand why empty paths were being passed in to the AddIncludes function.

While there is likely a problem elsewhere inserting the empty path, it seems reasonable to have this method be robust to bad input from plugins.

clang parser was getting called with commandline that looked like: "-I", "-I/path/to/dir"
Files located in "/path/to/dir" were not being found. Removing the "-I" fixed the problem.

Test Plan:
Not sure how to reproduce my situation currently, other than manually ensuring an empty path is set.

I did verify that clang compiler itself exhibits this behavior from the commandline.

Reviewers: kfunk, mwolff

Reviewed By: mwolff

Subscribers: andric, kdevelop-devel

Projects: KDevelop

Differential Revision: https://phabricator.kde.org/D450