Tests fixed

Author: VitaliyKurokhtin

src/Microsoft.OpenApi/Models/OpenApiResponse.cs

@@ -78,7 +78,14 @@ public void SerializeAsV3WithoutReference(IOpenApiWriter writer)
             writer.WriteStartObject();
 
             // description
-            writer.WriteRequiredProperty(OpenApiConstants.Description, Description);
+            if (this.Reference == null)
+            {
+                writer.WriteRequiredProperty(OpenApiConstants.Description, Description);
+            }
+            else
+            {
+                writer.WriteProperty(OpenApiConstants.Description, Description);
+            }
 
             // headers
             writer.WriteOptionalMap(OpenApiConstants.Headers, Headers, (w, h) => h.SerializeAsV3(w));
@@ -122,7 +129,15 @@ public void SerializeAsV2WithoutReference(IOpenApiWriter writer)
             writer.WriteStartObject();
 
             // description
-            writer.WriteRequiredProperty(OpenApiConstants.Description, Description);
+            if (this.Reference == null)
+            {
+                writer.WriteRequiredProperty(OpenApiConstants.Description, Description);
+            }
+            else
+            {
+                writer.WriteProperty(OpenApiConstants.Description, Description);
+            }
+
             if (Content != null)
             {
                 var mediatype = Content.FirstOrDefault();

src/Microsoft.OpenApi/Writers/OpenApiWriterExtensions.cs

@@ -42,7 +42,14 @@ public static void WriteRequiredProperty(this IOpenApiWriter writer, string name
         {
             CheckArguments(writer, name);
             writer.WritePropertyName(name);
-            writer.WriteValue(value);
+            if (value == null)
+            {
+                writer.WriteNull();
+            }
+            else
+            {
+                writer.WriteValue(value);
+            }
         }
 
         /// <summary>

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

@@ -359,6 +359,7 @@ public void SerializeOperationWithBodyAsV3JsonWorks()
       ""$ref"": ""#/components/responses/response1""
     },
     ""400"": {
+      ""description"": null,
       ""content"": {
         ""application/json"": {
           ""schema"": {
@@ -431,6 +432,7 @@ public void SerializeAdvancedOperationWithTagAndSecurityAsV3JsonWorks()
       ""$ref"": ""#/components/responses/response1""
     },
     ""400"": {
+      ""description"": null,
       ""content"": {
         ""application/json"": {
           ""schema"": {
@@ -554,7 +556,7 @@ public void SerializeOperationWithFormDataAsV3JsonWorks()
 
             // Act
             var actual = _operationWithFormData.SerializeAsJson(OpenApiSpecVersion.OpenApi3_0);
-            
+
             // Assert
             actual = actual.MakeLineBreaksEnvironmentNeutral();
             expected = expected.MakeLineBreaksEnvironmentNeutral();
@@ -607,7 +609,7 @@ public void SerializeOperationWithFormDataAsV2JsonWorks()
 
             // Act
             var actual = _operationWithFormData.SerializeAsJson(OpenApiSpecVersion.OpenApi2_0);
-            
+
             // Assert
             actual = actual.MakeLineBreaksEnvironmentNeutral();
             expected = expected.MakeLineBreaksEnvironmentNeutral();
@@ -658,6 +660,7 @@ public void SerializeOperationWithBodyAsV2JsonWorks()
       ""$ref"": ""#/responses/response1""
     },
     ""400"": {
+      ""description"": null,
       ""schema"": {
         ""maximum"": 10,
         ""minimum"": 5,
@@ -672,7 +675,7 @@ public void SerializeOperationWithBodyAsV2JsonWorks()
 
             // Act
             var actual = _operationWithBody.SerializeAsJson(OpenApiSpecVersion.OpenApi2_0);
-            
+
             // Assert
             actual = actual.MakeLineBreaksEnvironmentNeutral();
             expected = expected.MakeLineBreaksEnvironmentNeutral();
@@ -727,6 +730,7 @@ public void SerializeAdvancedOperationWithTagAndSecurityAsV2JsonWorks()
       ""$ref"": ""#/responses/response1""
     },
     ""400"": {
+      ""description"": null,
       ""schema"": {
         ""maximum"": 10,
         ""minimum"": 5,

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

@@ -116,11 +116,18 @@ public void SerializeBasicResponseWorks(
             OpenApiSpecVersion version,
             OpenApiFormat format)
         {
-            // Arrange & Act
+            // Arrange
+            var expected = format == OpenApiFormat.Json ? @"{
+  ""description"": null
+}" : @"description: ";
+
+            // Act
             var actual = BasicResponse.Serialize(version, format);
 
             // Assert
-            actual.Should().Be("{ }");
+            actual = actual.MakeLineBreaksEnvironmentNeutral();
+            expected = expected.MakeLineBreaksEnvironmentNeutral();
+            actual.Should().Be(expected);
         }
 
         [Fact]