JIRA: https://jira.spring.io/browse/INT-4043
The `ExecutorChannel` overrides `onInit()` but fails to call the super
which is where the message converter for datatype conversion is set up.
Also, when Jackson is not on the class path and there are no converters in the
context, the default integration conversion service is not registered.
The `DefaultDatatypeChannelMessageConverter` overwites its default conversion
service with this bean, unconditionally - setting it to null in this case.
Check for a null conversion service before replacing the default.
* Polishing according PR comments
dae1a01003