Fixes https://github.com/spring-projects/spring-integration/issues/3558
Kotlin lambdas mostly used to configure endpoints in DSL manner
are not really Java lambdas, but rather anonymous classes implementing
respective Java interfaces.
While in most cases such classes carry generic info for their method impls
properly in Java, it is somehow doesn't work well for `GenericHandler`
implemented by Kotlin lambdas
* Wrap provided `GenericHandler` in the `BaseIntegrationFlowDefinition.handle()`
into a Java lambda and call `handle()` recursively to carry an expected type to
the `LambdaMessageProcessor`
* Fix `LambdaMessageProcessor` to handle `ClassUtils.isKotlinUnit()` result of
an invocation as a `null` reply
**Cherry-pick to `5.4.x` & `5.3.x`**
c682309edb