fix: adjust logic to address manage_master_user_password variable bugs

[RoseSecurity]

what and why

• This PR fixes critical bugs created when the manage_master_user_password variable was introduced. The first bug caused a Terraform error where the master_password output attempted to join a null value when AWS was managing the password, failing with "Invalid function argument: cannot concatenate null values."
• These fixes ensure that when manage_master_user_password = null (default), random passwords are created and stored in SSM as expected, and when manage_master_user_password = true, AWS manages the password without attempting to expose it in Terraform outputs or store it in SSM, maintaining both backward compatibility and the intended security

bug

│ Error: Invalid function argument
│ 
│   on .terraform/modules/documentdb/outputs.tf line 7, in output "master_password":
│    7:   value       = var.manage_master_user_password != null ? join("", aws_docdb_cluster.default[*].master_password) : null
│     ├────────────────
│     │ while calling join(separator, lists...)
│     │ aws_docdb_cluster.default is tuple with 1 element
│ 
│ Invalid value for "lists" parameter: element 0 is null; cannot concatenate null values.

references

• Closes Master password output broken after setting manage_master_user_password #126
• Closes BC break from 0.30.0 to 0.30.1 #127
• Closes The output condition is wrong and giving errors #128

[RoseSecurity]

/terratest

[tdinunzio-trayport]

This looks good to me. Tested in my environment and works as expected.

[github-actions]

These changes were released in v0.30.2.