If a QBindable is created from a computed property, it is not possible
to actually set a value or a binding. If we try to do it anyway, we'd
get a crash. Thus we now check whether the function pointer is null
before invoking it.
Pick-to: 6.0
Task-number: QTBUG-87153
Change-Id: I5bedb9080ccf79d9b8166b80d5733d095ed76f8d
Reviewed-by: Lars Knoll <lars.knoll@qt.io>