Fix an assert when there are no plugins.
Fix an assert when there are no plugins.

Not a usual situation on devices, but common in some desktop configurations.

Reviewed-by: Jonas Rabbe

qtproject/qt-mobility,qtproject/qt-mobility,KDE/android-qt-mobility,kaltsi/qt-mobility,KDE/android-qt-mobility,enthought/qt-mobility,enthought/qt-mobility,tmcguire/qt-mobility,tmcguire/qt-mobility,KDE/android-qt-mobility,qtproject/qt-mobility,enthought/qt-mobility,qtproject/qt-mobility,kaltsi/qt-mobility,kaltsi/qt-mobility,enthought/qt-mobility,tmcguire/qt-mobility,enthought/qt-mobility,qtproject/qt-mobility,KDE/android-qt-mobility,enthought/qt-mobility,kaltsi/qt-mobility,qtproject/qt-mobility,kaltsi/qt-mobility,kaltsi/qt-mobility,tmcguire/qt-mobility,tmcguire/qt-mobility