[applet] Only set VolumeMonitor object if it's valid
Otherwise we get loads of spam like this:
qrc:/plasma/plasmoids/org.kde.plasma.volume/contents/ui/ListItemBase.qml:253:37: Unable to assign [undefined] to QPulseAudio::VolumeObject*
qrc:/plasma/plasmoids/org.kde.plasma.volume/contents/ui/ListItemBase.qml:253:37: Unable to assign [undefined] to QPulseAudio::VolumeObject*
qrc:/plasma/plasmoids/org.kde.plasma.volume/contents/ui/ListItemBase.qml:253:37: Unable to assign [undefined] to QPulseAudio::VolumeObject*