AddExtension only if Type is still unknown

Author: Youssef1313

src/Microsoft.OpenApi/Reader/V3/OpenApiSchemaDeserializer.cs

@@ -225,11 +225,22 @@ internal static partial class OpenApiV3Deserializer
                 {
                     if (bool.TryParse(n.GetScalarValue(), out var parsed) && parsed)
                     {
-                        // While we are dealing with v3 document, we are still using x-nullable extension here.
-                        // This is used only as a marker during deserialization to indicate that the schema is nullable.
-                        // When we do FinalizeDeserialization, we will modify the OpenApiSchema.Type to
-                        // include JsonSchemaType.Null (only if needed), and always remove the extension.
-                        o.AddExtension(OpenApiConstants.NullableExtension, _nullableTrueExtension);
+                        if (o.Type is not null && o.Type != 0)
+                        {
+                            o.Type |= JsonSchemaType.Null;
+                        }
+                        else
+                        {
+                            // While we are dealing with v3 document, we are still using x-nullable extension here.
+                            // This is used only as a marker during deserialization to indicate that the schema is nullable.
+                            // When we do FinalizeDeserialization, we will modify the OpenApiSchema.Type to
+                            // include JsonSchemaType.Null (only if needed), and always remove the extension.
+                            // The reason is that "nullable" property should only take effect if "Type" is set.
+                            // If we knew Type is set, we add "Null" to it.
+                            // Otherwise, it might be that it will be set later.
+                            // In this case, we add the marker extension, and check at the end of deserialization, and remove the extension.
+                            o.AddExtension(OpenApiConstants.NullableExtension, _nullableTrueExtension);
+                        }
                     }
                 }
             },