INT-3919: FTP: Allow null for Remote Directory

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

src/reference/asciidoc/ftp.adoc

@@ -191,6 +191,9 @@ Starting with _version 4.2_, you can specify `remote-directory-expression` inste
 you to dynamically determine the directory on each poll.
 e.g `remote-directory-expression="@myBean.determineRemoteDir()"`.
 
+Starting with _version 4.3_, the `remote-directory`/`remote-directory-expression` attributes can be omitted assuming `null`.
+In this case, according to the FTP protocol, the Client working directory is used as a default remote directory.
+
 Sometimes file filtering based on the simple pattern specified via `filename-pattern` attribute might not be sufficient.
 If this is the case, you can use the `filename-regex` attribute to specify a Regular Expression (e.g.
 `filename-regex=".*\.test$"`).
@@ -255,7 +258,10 @@ Here is an example that uses a custom Filter implementation.
 
 _Poller configuration notes for the inbound FTP adapter_
 
-The job of the inbound FTP adapter consists of two tasks: _1) Communicate with a remote server in order to transfer files from a remote directory to a local directory.__2) For each transferred file, generate a Message with that file as a payload and send it to the channel identified by the 'channel' attribute._ That is why they are called 'channel-adapters' rather than just 'adapters'.
+The job of the inbound FTP adapter consists of two tasks:
+_1) Communicate with a remote server in order to transfer files from a remote directory to a local directory._
+_2) For each transferred file, generate a Message with that file as a payload and send it to the channel identified by the 'channel' attribute._
+That is why they are called 'channel-adapters' rather than just 'adapters'.
 The main job of such an adapter is to generate a Message to be sent to a Message Channel.
 Essentially, the second task mentioned above takes precedence in such a way that *IF* your local directory already has one or more files it will first generate Messages from those, and *ONLY* when all local files have been processed, will it initiate the remote communication to retrieve more files.
 
@@ -407,6 +413,13 @@ If the `-dirs` option is included, each recursive directory is also returned as
 In this case, it is recommended that the `-1` is not used because you would not be able to determine files Vs.
 directories, which is achievable using the `FileInfo` objects.
 
+Starting with _version 4.3_, the `FtpSession` supports `null` for the `list()` and `listNames()` methods,
+therefore the `expression` attribute can be omitted.
+From Java perspective there are two new constructor without `expression` argument for convenience.
+The `null` for `LS` command is treated as an Client working directory according to the FTP protocol.
+The working directory can be set via the `FTPClient.changeWorkingDirectory()` function when you extend the
+`DefaultFtpSessionFactory` and implement `postProcessClientBeforeConnect()` callback.
+
 *get*
 
 _get_ retrieves a remote file and supports the following option:

src/reference/asciidoc/whats-new.adoc

@@ -114,6 +114,14 @@ See <<http-inbound>> for more information.
 A new factory bean is provided to simplify the configuration of Jsch proxies for SFTP.
 See <<sftp-proxy-factory-bean>> for more information.
 
+==== FTP Changes
+
+The `FtpSession` now supports `null` for the `list()` and `listNames()` method, since it is possible by the
+underlying FTP Client.
+With that the `FtpOutboundGateway` can now be configured without `remoteDirectory` expression.
+And the `<int-ftp:inbound-channel-adapter>` can be configured without `remote-directory`/`remote-directory-expression`.
+See <<ftp>> for more information.
+
 ==== Router Changes
 
 The `ErrorMessageExceptionTypeRouter` supports now the `Exception` superclass mappings to avoid duplication