Handle missing sids collections in role constructors

[somiljain2006]

Fixes jenkinsci/configuration-as-code-plugin#2811

Normalize null collection values to empty sets in role constructors to prevent failures when configuration fields are omitted.

Testing done

Add regression tests covering JCasC configurations without sids and role initialization with missing collection values.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[somiljain2006]

@basil @oleg-nenashev @AbhyudayaSharma Can you review this pr?

[oleg-nenashev]

The change looks about right. I haven't touched the code of this plugin for a while though