fix elasticsearch/user: allow ensure=absent without password

[milad-zanganeh]

Pull Request (PR) description
Removing a user previously failed because the elasticsearch::user define always required and passed a password, even when ensure => absent.

This PR fixes the issue by making the password parameter optional and improving the logic for handling user removal.

Related Issue
Fixes #1264

Changes:

• File: manifests/user.pp
  • Make password an Optional[String].
  • When ensure => absent, declare elasticsearch_user with ensure => absent and do not pass a password.
  • Add validation to enforce that a password must be provided when ensure => present.

[bastelfreak]

@milad-zanganeh thanks for the PR. Would it be possible that you add a test for this change?