In 4a9470e5

 there was introduced
specific isNull() implementation for QSvgIconEngine. But the existing
check was not sufficient for the scenarios where the QIcon is
constructed by deserialization from QDataStream (from previously
serialized valid svg QIcon).

We also need to check the svgBuffers as the previously serialized
svgFiles are silently ignored by QSvgIconEngine::read().

Change-Id: Id64ce233baead053c8b5ca61f1d416e46a8c6088
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: David Edmundson <davidedmundson@kde.org>