If a schema is nullable, read type as an array, and remove "x-nullable"

Author: MaggieKimani1

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -553,10 +553,6 @@ public void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version,
             writer.WriteProperty(OpenApiConstants.Deprecated, Deprecated, false);
 
             // extensions
-            if (Extensions.ContainsKey(OpenApiConstants.NullableExtension))
-            {
-                Extensions.Remove(OpenApiConstants.NullableExtension);
-            }
             writer.WriteExtensions(Extensions, OpenApiSpecVersion.OpenApi3_0);
 
             writer.WriteEndObject();

src/Microsoft.OpenApi/Reader/V31/OpenApiSchemaDeserializer.cs

@@ -182,7 +182,14 @@ internal static partial class OpenApiV31Deserializer
             },
             {
                 "nullable",
-                (o, n, _) => o.Nullable = bool.Parse(n.GetScalarValue())
+                (o, n, _) => 
+                {
+                    var nullable = bool.Parse(n.GetScalarValue());
+                    if (nullable) // if nullable, convert type into an array of type and null
+                    {
+                        o.Type = new string[]{o.Type.ToString(), OpenApiConstants.Null};
+                    }
+                }
             },
             {
                 "discriminator",
@@ -242,6 +249,13 @@ public static OpenApiSchema LoadSchema(ParseNode node, OpenApiDocument hostDocum
                 propertyNode.ParseField(schema, _openApiSchemaFixedFields, _openApiSchemaPatternFields);
             }
 
+            if (schema.Extensions.ContainsKey(OpenApiConstants.NullableExtension))
+            {
+                var type = schema.Type;                
+                schema.Type = new string[] {(string)type, OpenApiConstants.Null};
+                schema.Extensions.Remove(OpenApiConstants.NullableExtension);
+            }
+
             return schema;
         }
     }