concatPaths: process empty path1 correctly

Summary:
When path1 was empty (i.e. 'current directory'), concatPaths() used to
return an absolute path /path2 instead of just path2, which broke e.g.
KUrlCompletion in certain cases.

This commit makes concatPaths() return just path2 if path1 is empty.

Reviewers: Frameworks, anthonyfieroni, dfaure

Reviewed By: dfaure

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12490