Distinguish between empty properties and properties that are not set
Review Request #118645 - Created June 10, 2014 and submitted
This restores the behavior on KWin4: if I set an X property that doesn't have any data on a window, it's still information, so this makes the Xcb wrapper return an empty QByteArray that is not null. EffectWindow::readProperty() now returns an empty QByteArray constructed the same way as it was in KWin4. This fixes the bug https://bugs.kde.org/show_bug.cgi?id=335446 The blur effect is supposed to just apply to the whole window if a region is not passed in the property, and in that case the property is here but empty.