Fix PersistentValue::isEmpty()
Fix PersistentValue::isEmpty()

A PersistentValue can have a d pointer and still be
empty (because it contains an empty value).

This fixes an assertion when destructing the QML engine
and qmlscene finally doesn't crash on exit anymore :)

Change-Id: Idce2bb42f562526af8ec09d9062cce7ac825a8dc
Reviewed-by: Simon Hausmann <a975102812d593b3191088e3d0aeaeb6f11dbcf5@digia.com>

qmlc/qtdeclarative,matthewvogt/qtdeclarative,matthewvogt/qtdeclarative,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,matthewvogt/qtdeclarative,qmlc/qtdeclarative,qmlc/qtdeclarative,matthewvogt/qtdeclarative,mgrunditz/qtdeclarative-2d,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,matthewvogt/qtdeclarative,mgrunditz/qtdeclarative-2d,mgrunditz/qtdeclarative-2d,qmlc/qtdeclarative,qmlc/qtdeclarative,qmlc/qtdeclarative,qmlc/qtdeclarative,mgrunditz/qtdeclarative-2d,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,qmlc/qtdeclarative