chore: address more PR feedback

Author: MaggieKimani1

src/Microsoft.OpenApi/Extensions/OpenApiSerializableExtensions.cs

@@ -82,7 +82,7 @@ public static Task SerializeAsync<T>(
             Stream stream,
             OpenApiSpecVersion specVersion,
             OpenApiFormat format,
-            OpenApiWriterSettings? settings,
+            OpenApiWriterSettings? settings = null,
             CancellationToken cancellationToken = default)
             where T : IOpenApiSerializable
         {
@@ -107,7 +107,7 @@ public static Task SerializeAsync<T>(
         /// <param name="writer">The output writer.</param>
         /// <param name="specVersion">Version of the specification the output should conform to</param>
         /// <param name="cancellationToken">The cancellation token.</param>
-        public static Task SerializeAsync<T>(this T? element, IOpenApiWriter writer, OpenApiSpecVersion specVersion, CancellationToken cancellationToken = default)
+        public static Task SerializeAsync<T>(this T element, IOpenApiWriter writer, OpenApiSpecVersion specVersion, CancellationToken cancellationToken = default)
             where T : IOpenApiSerializable
         {
             Utils.CheckArgumentNull(element);
@@ -116,15 +116,15 @@ public static Task SerializeAsync<T>(this T? element, IOpenApiWriter writer, Ope
             switch (specVersion)
             {
                 case OpenApiSpecVersion.OpenApi3_1:
-                    element?.SerializeAsV31(writer);
+                    element.SerializeAsV31(writer);
                     break;
 
                 case OpenApiSpecVersion.OpenApi3_0:
-                    element?.SerializeAsV3(writer);
+                    element.SerializeAsV3(writer);
                     break;
 
                 case OpenApiSpecVersion.OpenApi2_0:
-                    element?.SerializeAsV2(writer);
+                    element.SerializeAsV2(writer);
                     break;
 
                 default:

src/Microsoft.OpenApi/Extensions/StringExtensions.cs

@@ -30,7 +30,7 @@ internal static class StringExtensions
             return false;
 
         }
-        internal static bool TryGetEnumFromDisplayName<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicFields)] T>(this string? displayName, out T result) where T : Enum
+        internal static bool TryGetEnumFromDisplayName<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicFields)] T>(this string? displayName, out T? result) where T : Enum
         {
             var type = typeof(T);
 
@@ -42,7 +42,7 @@ internal static class StringExtensions
                 return true;
             }
 
-            result = default!;
+            result = default;
             return false;
         }
         private static ReadOnlyDictionary<string, object> GetEnumValues<T>([DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicFields)] Type enumType) where T : Enum

src/Microsoft.OpenApi/Reader/OpenApiJsonReader.cs

@@ -88,9 +88,9 @@ public ReadResult Read(JsonNode jsonNode,
             }
 
             // Validate the document
-            if (settings.RuleSet != null && settings.RuleSet.Rules.Any())
+            if (document is not null && settings.RuleSet is not null && settings.RuleSet.Rules.Any())
             {
-                var openApiErrors = document?.Validate(settings.RuleSet);
+                var openApiErrors = document.Validate(settings.RuleSet);
                 if(openApiErrors is not null)
                 {
                     foreach (var item in openApiErrors.OfType<OpenApiValidatorError>())
@@ -200,9 +200,9 @@ public async Task<ReadResult> ReadAsync(Stream input,
             }
 
             // Validate the element
-            if (settings.RuleSet != null && settings.RuleSet.Rules.Any())
+            if (element is not null && settings.RuleSet is not null && settings.RuleSet.Rules.Any())
             {
-                var errors = element?.Validate(settings.RuleSet);
+                var errors = element.Validate(settings.RuleSet);
                 if (errors is not null)
                 {
                     foreach (var item in errors)