fix: Correct evaluation logic when determining master_user_options arguments

[dim-ops]

Description

#18

How Has This Been Tested?

advanced_security_options = {
    enabled                        = true
    anonymous_auth_enabled         = false
    internal_user_database_enabled = true

    master_user_options = {
      master_user_name     = "admin"
      master_user_password = "PasswoRd12-"
    }
  }

advanced_security_options = {
    enabled                        = true
    anonymous_auth_enabled         = false
    internal_user_database_enabled = false

    master_user_options = {
      master_user_arn     = aws_iam_user.opensearch_cluster.arn
    }
  }

• Resolves can't use iam user as master_user_arn #18
• Resolves Can't use master_user_name if don't set master_user_arn to null explicitly #19

[velkovb]

I think that will fix #19. I was planning a PR but you got it first :)

[antonbabenko]

This PR is included in version 1.3.1 🎉

[github-actions]

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