When we use explicitly `new NullChannel()` in DSL definition, we don't
provide a bean name and just end up with an explicit `nullChannel`
component name in the class.
This way it clashes with the global `errorChannel`, but since instances
are different we get a `BeanDefinitionOverrideException`
* Remove explicit `nullChannel` component name in the `NullChannel` in
favor of the generated name as it is done for any regular beans
* Some internal polishing in the `NullChannel`
* Some polishing in the `IntegrationFlowBeanPostProcessor` to deal with
`NullChannel` as with any other regular `AbstractMessageChannel`s and
generate a proper name for it within the scope of the current
`IntegrationFlow`
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