Don't try to reload QQuick images when changing to null screen

Don't try to reload QQuick images when changing to null screen

When a QGuiApplication is destructing, a screen change occurs causing
QQuickImageBase to reload images ultimately resulting in "QPixmap: Must
construct a QGuiApplication before a QPixmap". This patch fixes the
issue (except for systems which multiple screens, which is a separate
issue).

Task-number: QTBUG-40539
Change-Id: I4bafc9790741204e90190a4491ac7a9393d0d0a7
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>