Major Description
In the case if user has declared empty password server does not allow to connect user.
Empty password does not processed for normal user and "anonymous" and for all encrypted modes.
Sources of problem.
1. command PASS blocked using empty password since in this case request contains NULL instead password value and command generates error 501
2. In the case of using properties way of user management
the class PropertiesUserManager, for case of configured empty password, makes decision that user does not exist.