JIRA: https://jira.spring.io/browse/INT-3919
Since `FtpClient` supports `null` for the `LS` command, treating it as a current `working directory`,
there is no reason to forbid `null` from the FTP adapters end-user perspective.
* Allow `null` for the `FtpSession` `list()` and `listNames()` methods
* Allow `null` in the `remote-directory` for the `<int-ftp:inbound-channel-adapter>`
* Allow `null` in the `expression` for the `FtpOutboundGateway`
Polishing - send error if error on async output
Cover `onFailure()` from `onSuccess()` with the `errorChannel`
Address PR Comments
* Get rid of `null` population for the `remoteDirectoryExpression` in the `AbstractPollingInboundChannelAdapterParser`
* Populate `new LiteralExpression(null)` from the `FtpInboundFileSynchronizer` ctor
* Introduce `buildRemotePath(parent, child)` function in the `AbstractRemoteFileOutboundGateway` with the `null` logic for `parent`
* Rework `mGetWithoutRecursion()` to use `LS` command and allow `null` for the dir.
* Fix tests according the new `mGetWithoutRecursion()` logic
Polishing
7595e4f142