QFontMetrics throws exception when passing empty QFont

When an empty `QFont` is passed to `QFontMetrics`, calling 'QFontMetrics::width' throws an exception. Apparently this exception is only thrown in Windows.

Here's a minimal working example that triggers a crash on Windows (Qt Creator 4.6.2, Qt 5.9.6, msvc2017)

#include <QApplication>
#include <QFontMetrics>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QFont font("", 0);
    QFontMetrics fm(font);

    QString text("testing");
    auto width = fm.width(text);

    return a.exec();
}