* GH-3452: Warn Publisher.onError in NullChannel
Fixes https://github.com/spring-projects/spring-integration/issues/3452
The `NullChannel` can subscribe to the `Publisher` payload fully ignoring
the possible data according the `NullChannel` nature.
However an error thrown from the reactive stream processing is also ignored.
* Log WARN message from the `Subscriber.onError()` when `NullChannel`
subscribes to the `Publisher` produced to this channel.
* Mention the logic in the `NullChannel` docs; point to the
`ReactiveRequestHandlerAdvice` for further possible error handling in
the target application
**Cherry-pick to `5.4.x`**
* * Remove unused imports
* * Fix `PseudoTransactionalMessageSourceTests` for `LogAccessor`
from `NullChannel`
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