Emit proper compile error while trying to #[export] Gd<T> or DynGd<T, D>.

Author: Yarwin

godot-macros/src/class/derive_godot_class.rs

@@ -680,6 +680,14 @@ fn parse_fields(
         if let Some(mut parser) = KvParser::parse(&named_field.attributes, "export")? {
             let export = FieldExport::new_from_kv(&mut parser)?;
             field.export = Some(export);
+
+            if path_ends_with_complex(&field.ty, "Gd") || path_ends_with_complex(&field.ty, "DynGd")
+            {
+                return bail!(
+                    field.ty,
+                    "`Gd` and `DynGd` cannot be used directly as properties because they are non-nullable. Use `OnEditor<T>` or `Option<T>` instead."
+                );
+            }
             parser.finish()?;
         }