[backport v2.9.3] Default to empty object for missing machineSelectorConfig

[github-actions]

This is an automated request to port PR #12061 by @rak-phillip

Original PR body:

Summary

This defaults to an empty object when machineSelectorConfig is missing from the RKE2 manifest. An unhandled exception would throw if machineSelectorConfig is undefined, causing the edit form to fail on render.

Fixes #12058

Occurred changes and/or fixed issues

• Use optional chaining when when attempting to access machineSelectorConfig in the agenConfig getter
• Default to an empty object if the lookup fails

Areas or cases that should be tested

• This can happen when creating custom RKE2 clusters

Areas which could experience regressions

• Notes on how to reproduce are in the issue

Checklist

• The PR is linked to an issue and the linked issue has a Milestone, or no issue is needed
• The PR has a Milestone
• The PR template has been filled out
• The PR has been self reviewed
• The PR has a reviewer assigned
• The PR has automated tests or clear instructions for manual tests and the linked issue has appropriate QA labels, or tests are not needed
• The PR has reviewed with UX and tested in light and dark mode, or there are no UX changes