Modify New-AzVmss Vmss Flex defaulting logic to avoid null reference errors

[Sandido]

Description

New-AzVmss when used with a VmssConfig object with OrchestrationMode set (Vmss Flex) hits a null reference error.
This was due to an incorrect usage of the c# syntax 'object?.property?.property2 == null' if check. This caused null to be returned not only when property2 was null, but when property was null. When the code in the if check then referred to property, a null reference occurred.

This update will prevent this issue by adding an explicit if check for object?.property != null before proceeding with the code.

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate ChangeLog.md file(s) has been updated:
  • For any service, the ChangeLog.md file can be found at src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md
  • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header -- no new version header should be added
• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • a cmdlet design review was approved for the changes in this repository (Microsoft internal only)
    • https://github.com/Azure/azure-powershell-cmdlet-review-pr/issues/1158
  • the markdown help files have been regenerated using the commands listed here