[SQL] Fix null ref when not logged into Azure account for all sql cmdlets

[adeal]

Description

Current behavior of SQL cmdlets inheriting from AzureSqlCmdletBase results in the user getting a nullref exception when using a command prior to logging into their AzAccount. This is caused by the existing code unconditionally accessing DefaultProfile.DefaultContext.Subscription, which only gets set upon login.

This change fixes this behavior by:

1. Change the signature of AzureSqlCmdletBase.InitModelAdapter() to no longer have a IAzureSubscription parameter
2. Conditionally access IAzureContext.Subscription

Existing behavior:

New behavior:

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate change log has been updated
• 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:
  • the changes have gone through a cmdlet design review (Microsoft internal partners only)
  • the cmdlet markdown files were generated using the platyPS module

[jaredmoo]

Thank you!

[vladimir-shcherbakov]

signing: https://azuresdkci.westus2.cloudapp.azure.com/view/PowerShell/job/psnetsign-esrp/114/