QIcon::isNull() returns false even when QIcon constructed with non-existent file

The QIcon::isNull() function simply returns whether or not the icon contains a valid d-pointer. This causes problems because if the QIcon constructor (takes a filename) or the addFile() function is called with an invalid filename, then the d-pointer gets initialized anyway even though the file doesn't exist. This differs from the behavior of classes like QPixmap and QImage which will be null if the file does not exist.