Only serialize window pixmap if we're using it

The previous QIcon::name() check wasn't sufficient as it only works with theme icon names (e.g. kate)
but not icons created from an absolute path, e.g. /some/special/icon/location/kate.png.
The latter is usually the case for containerized apps which have a proper application desktop file
installed but their icon in some path within the application image or some container daemon location.
Since we already store the information for whether we had to fall back to using the actual window pixmap,
check for this before trying to serialize icon pixmap data.

Differential Revision: https://phabricator.kde.org/D25762