Don't show "Open With" menu items for empty directories

Don't show "Open With" menu items for empty directories

Summary:
Any app that registers itself as able to open directories generally can't do anything
useful with an empty directory. So, don't show the {nav Open With} menu items for them.

Test Plan:
Before: {F6759793}

After: {F6759794}

The {nav Open With} items still appear for non-empty directories.

Reviewers: #dolphin, elvisangelaccio

Reviewed By: #dolphin, elvisangelaccio

Subscribers: trmdi, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D20396