[AppService]: fix #19592- 'Import-AzWebAppKeyVaultCertificate' 'Object reference' error when importing a certificate previously imported via Azure Portal (#20424)

* [AppService]: fix #19592- 'Import-AzWebAppKeyVaultCertificate' 'Object reference' error when importing a certificate previously imported via Azure Portal

* updated ChangeLog.md

* Updated review comments

Author: Kotasudhakarreddy

src/Websites/Websites.Test/ScenarioTests/CertificatesTests.ps1

@@ -155,7 +155,7 @@ function Test-ImportAzWebAppKeyVaultCertificate
 	{		
 		#Setup
 		$kvcert = Import-AzWebAppKeyVaultCertificate -ResourceGroupName $rgname -WebAppName $wname -KeyVaultName $keyvaultname -CertName $keyvaultcertname
-		$default_webapp_certname= $rgname+'-'+$keyvaultname+'-'+$keyvaultcertname
+		$default_webapp_certname= $keyvaultname+'-'+$keyvaultcertname
 		# Assert
 		Assert-AreEqual $kvcert.Name $default_webapp_certname