Add constants

Author: MaggieKimani1

src/Microsoft.OpenApi/Models/OpenApiConstants.cs

@@ -700,6 +700,16 @@ public static class OpenApiConstants
         /// </summary>
         public const string ComponentsSegment = "/components/";
 
+        /// <summary>
+        /// Field: Null
+        /// </summary>
+        public const string Null = "null";
+
+        /// <summary>
+        /// Field: Nullable extension
+        /// </summary>
+        public const string NullableExtension = "x-nullable";
+
         #region V2.0
 
         /// <summary>

test/Microsoft.OpenApi.Tests/PublicApi/PublicApi.approved.txt

@@ -437,7 +437,9 @@ namespace Microsoft.OpenApi.Models
         public const string Name = "name";
         public const string Namespace = "namespace";
         public const string Not = "not";
+        public const string Null = "null";
         public const string Nullable = "nullable";
+        public const string NullableExtension = "x-nullable";
         public const string OneOf = "oneOf";
         public const string OpenApi = "openapi";
         public const string OpenIdConnectUrl = "openIdConnectUrl";