fix the null reference if the collection in Operation being set as null

Author: xuzhg

src/Microsoft.OpenApi/Models/OpenApiOperation.cs

@@ -199,7 +199,15 @@ public void SerializeAsV2(IOpenApiWriter writer)
             // operationId
             writer.WriteProperty(OpenApiConstants.OperationId, OperationId);
 
-            var parameters = new List<OpenApiParameter>(Parameters);
+            IList<OpenApiParameter> parameters;
+            if (Parameters == null)
+            {
+                parameters = new List<OpenApiParameter>();
+            }
+            else
+            {
+                parameters = new List<OpenApiParameter>(Parameters);
+            }
 
             if (RequestBody != null)
             {
@@ -258,7 +266,9 @@ public void SerializeAsV2(IOpenApiWriter writer)
             // All schemes in the Servers are extracted, regardless of whether the host matches
             // the host defined in the outermost Swagger object. This is due to the 
             // inaccessibility of information for that host in the context of an inner object like this Operation.
-            var schemes = Servers.Select(
+            if (Servers != null)
+            {
+                var schemes = Servers.Select(
                     s =>
                     {
                         Uri.TryCreate(s.Url, UriKind.RelativeOrAbsolute, out var url);
@@ -268,7 +278,8 @@ public void SerializeAsV2(IOpenApiWriter writer)
                 .Distinct()
                 .ToList();
 
-            writer.WriteOptionalCollection(OpenApiConstants.Schemes, schemes, (w, s) => w.WriteValue(s));
+                writer.WriteOptionalCollection(OpenApiConstants.Schemes, schemes, (w, s) => w.WriteValue(s));
+            }
 
             // deprecated
             writer.WriteProperty(OpenApiConstants.Deprecated, Deprecated, false);

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

@@ -26,7 +26,7 @@ public OpenApiOperationTests(ITestOutputHelper output)
         {
         };
 
-        public static OpenApiOperation AdvancedOperation= new OpenApiOperation
+        public static OpenApiOperation AdvancedOperation = new OpenApiOperation
         {
             Summary = "summary1",
             Description = "operationDescription",
@@ -61,7 +61,7 @@ public OpenApiOperationTests(ITestOutputHelper output)
                         {
                             Type = "number",
                             Minimum = 5,
-                            Maximum =  10
+                            Maximum = 10
                         }
                     }
                 }
@@ -229,7 +229,7 @@ public void SerializeBasicOperationAsV3JsonWorks()
 }";
 
             // Act
-                var actual = BasicOperation.SerializeAsJson(OpenApiSpecVersion.OpenApi3_0_0);
+            var actual = BasicOperation.SerializeAsJson(OpenApiSpecVersion.OpenApi3_0_0);
 
             _output.WriteLine(actual);
 
@@ -308,7 +308,7 @@ public void SerializeAdvancedOperationAsV3JsonWorks()
             expected = expected.MakeLineBreaksEnvironmentNeutral();
             actual.Should().Be(expected);
         }
-        
+
         [Fact]
         public void SerializeAdvancedOperationWithTagAndSecurityAsV3JsonWorks()
         {
@@ -555,5 +555,27 @@ public void SerializeAdvancedOperationWithTagAndSecurityAsV2JsonWorks()
             expected = expected.MakeLineBreaksEnvironmentNeutral();
             actual.Should().Be(expected);
         }
+
+        [Fact]
+        public void SerializeOperationWithNullCollectionAsV2JsonWorks()
+        {
+            // Arrange
+            var expected = @"{
+  ""responses"": { }
+}";
+            OpenApiOperation operation = new OpenApiOperation
+            {
+                Parameters = null,
+                Servers = null,
+            };
+
+            // Act
+            var actual = operation.SerializeAsJson(OpenApiSpecVersion.OpenApi2_0);
+
+            // Assert
+            actual = actual.MakeLineBreaksEnvironmentNeutral();
+            expected = expected.MakeLineBreaksEnvironmentNeutral();
+            actual.Should().Be(expected);
+        }
     }
 }