[Task Manager] Only match application name if we never managed to match PID

When there are two instances of the same application running, one playing audio
and the other one not, it will errneously show an indicator on both applications:
for the playing one it will find a PID but for the other one it won't and then
try appName matching which will then result in the same stream.

Differential Revision: https://phabricator.kde.org/D4038