EventHandler attributes are nullable callback function attributes with
[LegacyTreatNonObjectAsNull]. Assigning a non-object value should
produce null, but assigning an object value should preserve that object
as the callback value, even when the object is not callable.
Previously, object values such as `{ handleEvent() {} }` could pass the
nullable conversion and then be converted to null by the inner callback
function conversion. This made MessagePort.onmessage in the fixed test
return null after such an assignment.
Handle the non-object-to-null rule in nullable conversion, and let the
legacy callback conversion wrap object values without rejecting them for
not being callable.
Fixes a regression in the python port of the IDL generator.
cc871620e6