The QML engine might (in the future) actually send the right message
to indicate a JavaScript null, that is:

{ type: "object", value: null }

This piece of JSON is then transformed into a QVariantMap. The QVariant
that signifies the null is unfortunately different across various Qt
versions. We can, however, detect it.

Change-Id: I3db42bb35a936bc02c749ab3a136b1d297aefdfe
Reviewed-by: hjk <hjk@qt.io>