Remove ValidateNotNullOrEmpty validation conditions for SubjectBeginsWith/SubjectEndsWith in Update-AzureRmEventGridSubscription cmdlet to allow changing these parameters to empty string if needed

[ahamad-MS]

Description

This PR fixes #6331. In this issue, customer has a need to update parameters (SubjectBeginsWith) or (SubjectEndsWith) in Update-AzureRmEventGridSubscription cmdlet with empty string. ValidateNotNullOrEmpty validation on these parameters disallows this operation and needs to be removed as they are preventing legitimate operation.

Checklist

• [X ] I have read the Submitting Changes section of CONTRIBUTING.md
• [ X] The title of the PR is clear and informative
• [X ] The appropriate change log has been updated
• [ X] The PR does not introduce breaking changes
• [X ] 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
  • the cmdlet markdown files were generated using the platyPS module

[maddieclayton]

@ahamad-MS Do we want to remove this empty check on New-AzureRmEventGridSubscription as well?

[ahamad-MS]

@maddieclayton .. the check for nullandempty is still needed for the New-AzureRmEventGridSubscription cmdlet only the update command needs to be changed to allow user to update this parameter. Hence no change in the New- cmdlet since we want the user to specify this value if this optional parameter is chosen. Otherwise, it should be specified

[maddieclayton]

Sounds good.

[maddieclayton]

on demand here: https://azuresdkci.westus2.cloudapp.azure.com/view/PowerShell/job/powershell-demand/921/