Make QVariant conversion for JS null type symmetric

Make QVariant conversion for JS null type symmetric

If you pass (void*)0 to QJSEngine::toScriptValue(), or you pass a
QVariant of type QMetaType::VoidStar containing a 0 value, you get
back a QJSValue of type null (isNull() returns true); that's fine.

However, if you called QJSValue::toVariant() on a JS null value, you
would get back an invalid QVariant. The expected result is a
QVariant of type QMetaType::VoidStar containing a 0 value. This
makes the conversion of the JS null type symmetric and avoids loss
of data.

Change-Id: Ifa6e788152118f80adf9c2d7be1283f053b44294
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@nokia.com>
Reviewed-by: Simon Hausmann <simon.hausmann@nokia.com>