patch to fix endless looping of KDE audio notifications

phonon-backend-gstreamer (using phonon-4.6.0) is apparently unable to tell when
the source queue contains no more sources.  In particular, when using
phonon-backend-gstreamer as the backend for playing KDE audio notifications, on
login, it loops forever playing the KDE login jingle over and over as reported
above.  Because of this, phonon-backend-gstreamer from git is entirely unusable
as a KDE audio backend.

Looking at the commit, and specifically at gstreamer/mediaobject.cpp, it
appears that Phonon sends and empty source when there is no next source, but in
MediaObject::setNextSource() and MediaObject::handleAboutToFinish() this case
is not handled and m_skippingEOS is always set to true, resulting in the
current source being played over and over again.

The attached patch works around this by detecting if Phonon has sent and empty
(or invalid) source and then setting m_skippingEOS appropriately.  This not be
the proper way of doing this, but I hope at least that it sheads light on the
issue.

BUG:292411