Mail: Proposal for removing constructors in AbstractMailReceiver which support url to be null

Back-port of: https://github.com/spring-projects/spring-integration/issues/10100Back-port of: https://github.com/spring-projects/spring-integration/issues/10100

Fixed via: https://github.com/spring-projects/spring-integration/commit/44684a2e6840a95728374b3cd35501b51be0c9ab