Fixes: #9427 Issue link: https://github.com/spring-projects/spring-integration/issues/9427 The `expression` of the `AbstractRemoteFileOutboundGateway` could be `null` and ignored for some expressions making a DSL factories inconsistent * Improve `AbstractRemoteFileOutboundGateway` constructor JavaDocs explaining `expression` argument in more details * Add `@Nullable` for this `expression` arg in all the `AbstractRemoteFileOutboundGateway` implementations * Add new DSL factory method for `outboundGateway()` without this `expression` for those commands when it is not needed