xdg-email: avoid passing empty kreadconfig return to desktop_file_to_binary

Fixes a warning from grep when the default MUA is unset in KDE 6:

grep: /home/foo/.local/share/applications//: Is a directory

added 20 commits

• c27e7e64...23a1207c - 18 commits from branch xdg:master
• 71a7dbeb - xdg-email: avoid passing empty kreadconfig return to desktop_file_to_binary
• 803b307e - Ignore empty arguments to binary_to_desktop_file and desktop_file_to_binary

Compare with previous version

merged

Thanks sorry I didn't get to this sooner.

Thanks for merging, and thanks for all the other recent work, much appreciated.

PS: not sure why you squashed the branch, it had two logical commits (with different commit messages) that should have been kept separate.