KRun: fix assert when failing to start an application

Test Plan:
'sudo chmod a-x /usr/bin/gwenview'
Try opening a picure file from e.g. dolphin, you get an error message as expected, clicking OK makes dolphin crash because m_pids is empty and we hit an assert in QVector

Reviewers: ahmadsamir

Reviewed By: ahmadsamir

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28598