libkmpris: don't return a PlayerContainer for non-running apps
If we ask for the playerData of an app with a pid of 0, which doesn't make sense, we get an answer that doesn't make sense: the Task Manager shows the player controls for launchers.
If we ask for the playerData of an app with a pid of 0, which doesn't make sense, we get an answer that doesn't make sense: the Task Manager shows the player controls for launchers.