Clarify NPE when resolveScm omits targets

[Chikzz2001]

When targets is omitted from resolveScm, Pipeline DataBoundConstructor binding passes null and new ArrayList<>(null) throws a cryptic NPE (JENKINS-67005 / jenkinsci/github-branch-source-plugin#1380).

Fail fast with a clear message that targets is required.

Testing done

• Added unit test covering null targets
• Existing ResolveScmStepTest scenarios unchanged

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

Made with Cursor