Fix range field order (#56)

* Skip validate_arguments when empty

* Fix `range` field order

* Fix unused variable

parser/src/python.lalrpop

@@ -956,18 +956,16 @@ FuncDef: ast::Stmt = {
 
 Parameters: ast::Arguments = {
     <location:@L> "(" <a: (ParameterList<TypedParameter, StarTypedParameter>)?> ")" <end_location:@R> =>? {
-        let args = validate_arguments(
-            a.unwrap_or_else(|| ast::Arguments {
-                posonlyargs: vec![],
-                args: vec![],
-                vararg: None,
-                kwonlyargs: vec![],
-                kw_defaults: vec![],
-                kwarg: None,
-                defaults: vec![],
-                range: optional_range(location, end_location)
-            })
-        )?;
+        let args = a.map(validate_arguments).transpose()?.unwrap_or_else(|| ast::Arguments {
+            posonlyargs: vec![],
+            args: vec![],
+            vararg: None,
+            kwonlyargs: vec![],
+            kw_defaults: vec![],
+            kwarg: None,
+            defaults: vec![],
+            range: optional_range(location, end_location)
+        });
 
         Ok(args)
     }