resolve error

Author: Luyunmt

sdk/core/Azure.Core.TestFramework/src/TestEnvironment.cs

@@ -131,7 +131,7 @@ static TestEnvironment()
         /// <summary>
         ///   The URL of the Azure Authority host to be used for authentication. Recorded.
         /// </summary>
-        public string AuthorityHostUrl => GetRecordedOptionalVariable("AZURE_AUTHORITY_HOST");
+        public string AuthorityHostUrl => GetRecordedOptionalVariable("AZURE_AUTHORITY_HOST") ?? "https://login.microsoftonline.com/";
 
         /// <summary>
         ///   The suffix for Azure Storage accounts for the active cloud environment, such as "core.windows.net".  Recorded.