remove optional properties initializer

src/Microsoft.OpenApi.Readers/V2/OpenApiDocumentDeserializer.cs

@@ -93,6 +93,11 @@ public static OpenApiDocument LoadOpenApi(RootNode rootNode)
             ReportMissing(openApiNode, required);
 
             // Post Process OpenApi Object
+            if (openApidoc.Servers == null)
+            {
+                openApidoc.Servers = new List<OpenApiServer>();
+            }
+
             MakeServers(openApidoc.Servers, openApiNode.Context);
 
             return openApidoc;

src/Microsoft.OpenApi/Models/OpenApiDocument.cs

@@ -30,7 +30,7 @@ public class OpenApiDocument : OpenApiElement, IOpenApiExtension
         /// <summary>
         /// An array of Server Objects, which provide connectivity information to a target server.
         /// </summary>
-        public IList<OpenApiServer> Servers { get; set; } = new List<OpenApiServer>();
+        public IList<OpenApiServer> Servers { get; set; }
 
         /// <summary>
         /// REQUIRED. The available paths and operations for the API.
@@ -50,12 +50,12 @@ public class OpenApiDocument : OpenApiElement, IOpenApiExtension
         /// <summary>
         /// A list of tags used by the specification with additional metadata.
         /// </summary>
-        public IList<OpenApiTag> Tags { get; set; } = new List<OpenApiTag>();
+        public IList<OpenApiTag> Tags { get; set; }
 
         /// <summary>
         /// Additional external documentation.
         /// </summary>
-        public OpenApiExternalDocs ExternalDocs { get; set; } = new OpenApiExternalDocs();
+        public OpenApiExternalDocs ExternalDocs { get; set; }
 
         /// <summary>
         /// This object MAY be extended with Specification Extensions.

src/Microsoft.OpenApi/Services/OpenApiWalker.cs

@@ -32,12 +32,16 @@ public void Walk(OpenApiDocument doc)
             this._visitor.Visit(doc);
             this._visitor.Visit(doc.Info);
             this._visitor.Visit(doc.Servers);
-            foreach (var server in doc.Servers)
+
+            if (doc.Servers != null)
             {
-                this._visitor.Visit(server);
-                foreach (var variable in server.Variables.Values)
+                foreach (var server in doc.Servers)
                 {
-                    this._visitor.Visit(variable);
+                    this._visitor.Visit(server);
+                    foreach (var variable in server.Variables.Values)
+                    {
+                        this._visitor.Visit(variable);
+                    }
                 }
             }
 

src/Microsoft.OpenApi/Writers/OpenApiWriterExtensions.cs

@@ -133,7 +133,10 @@ public static void WriteRequiredObject<T>(this IOpenApiWriter writer, string nam
             CheckArguments(writer, name, action);
 
             writer.WritePropertyName(name);
-            action(writer, value);
+            if (value != null)
+            {
+                action(writer, value);
+            }
         }
 
         /// <summary>

test/Microsoft.OpenApi.Readers.Tests/ModelToV2Tests.cs

@@ -42,7 +42,11 @@ private static JObject ExportV2ToJObject(OpenApiDocument openApiDoc)
         [Fact]
         public void HostTest()
         {
-            var openApiDoc = new OpenApiDocument();
+            var openApiDoc = new OpenApiDocument
+            {
+                Servers = new List<OpenApiServer>()
+            };
+
             openApiDoc.Servers.Add(new OpenApiServer {Url = "http://example.org/api"});
             openApiDoc.Servers.Add(new OpenApiServer {Url = "https://example.org/api"});