Fix roundtrip conversion between JS var and QVariant

Fix roundtrip conversion between JS var and QVariant

Always convert null to a QVariant(VoidStar) as documented in
QJSValue. Make sure the reverse conversion will lead back to
a null JS value.

Adjusted two test cases that expected an invalid QVariant when
setting the property to null, and added test cases for the
correct conversion.

Task-number: QTBUG-40880
Change-Id: I6eb01f0067f2c89779c53fd2cd0a1193047ed2cc
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>