JIRA: https://jira.spring.io/browse/INT-4210
Previously use-case like:
```
.transform()
.log()
.get();
```
failed with the `Dispatcher has no subscribers` for an implicit channel populated by the `.wireTap()` because actually `LoggingHandler` is placed on the wire-tapped channel
Since logically it looks like end-user tries to have `LoggingHandler` as a terminal of the flow, we add `nullChannel` if `WireTapSpec` is in the end of the flow definition.
If user would like to continue flow (e.g. return reply to the `replyChannel`), he must end more EIP-methods after `.log()` (like before, of course)
36f3c533aa