Fix: Handle nil deployment outputs gracefully

[arunpnair]

What does this change

In ARM templates, the outputs element is used to return values from deployed resources, and its inclusion is optional. However, the current version of the arm-mixin incorrectly treats the outputs section as mandatory, generating an error when the ARM template being deployed does not define outputs section.

What issue does it fix

This pull request resolves the issue of arm-mixin deployment failures that occur when the ARM template lacks the outputs section. Furthermore, unit test cases have been added to ensure comprehensive coverage of all relevant aspects related to this fix.

Checklist

• Did you write tests?
• Did you write documentation?
• Did you change porter.yaml or a storage document record? Update the corresponding schema file.
• If this is your first pull request, please add your name to the bottom of our Contributors list. Thank you for making Porter better! 🙇‍♀️

[schristoff]

Thank you and thanks for adding test cases :)