KRun: fix runtime warning when widget is null
I wanted the connection to be broken if the widget was deleted early,
but if there's no widget then Qt warns about a null receiver. Use qApp
as receiver in that case, just to have something non-null.
Test Plan: kde-open5 file.pdf
Reviewers: nicolasfella, vkrause
Reviewed By: vkrause
Differential Revision: https://phabricator.kde.org/D23147