test(models): update response serialization expectation

Co-authored-by: baywet <7905502+baywet@users.noreply.github.com>

Author: Copilot

test/Microsoft.OpenApi.Tests/Models/OpenApiResponseTests.cs

@@ -193,9 +193,7 @@ public async Task SerializeBasicResponseWorks(
             string format)
         {
             // Arrange
-            var expected = format == OpenApiConstants.Json ? @"{
-  ""description"": null
-}" : @"description: null";
+            var expected = @"{ }";
 
             // Act
             var actual = await BasicResponse.SerializeAsync(version, format, TestContext.Current.CancellationToken);