UI fix: hide empty "External Scripts" submenu
Summary:
Steps to reproduce:
- show context menu for some Project Item
- select "External Scripts" - no action as a result
Reviewers: KDevelop, kfunk
Reviewed By: KDevelop, kfunk
Subscribers: kfunk, kdevelop-devel
Differential Revision: https://phabricator.kde.org/D3014