Silence qml error message

Summary:
model.AppPid is documented to not always exist, so use the same check
that is used in othere places in the file.
file:///usr/share/plasma/plasmoids/org.kde.plasma.taskmanager/contents/ui/Task.qml:347: Unable to assign [undefined] to int

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22207