Treat class C: ... and class C(): ... equivalently (#8659)

## Summary

These should be seen as identical from the `ComparableAst` perspective.

crates/ruff_python_ast/src/comparable.rs

@@ -363,7 +363,7 @@ impl<'a> From<&'a ast::Number> for ComparableNumber<'a> {
     }
 }
 
-#[derive(Debug, PartialEq, Eq, Hash)]
+#[derive(Debug, Default, PartialEq, Eq, Hash)]
 pub struct ComparableArguments<'a> {
     args: Vec<ComparableExpr<'a>>,
     keywords: Vec<ComparableKeyword<'a>>,
@@ -1051,7 +1051,7 @@ pub struct StmtClassDef<'a> {
     decorator_list: Vec<ComparableDecorator<'a>>,
     name: &'a str,
     type_params: Option<ComparableTypeParams<'a>>,
-    arguments: Option<ComparableArguments<'a>>,
+    arguments: ComparableArguments<'a>,
     body: Vec<ComparableStmt<'a>>,
 }
 
@@ -1309,7 +1309,7 @@ impl<'a> From<&'a ast::Stmt> for ComparableStmt<'a> {
                 range: _,
             }) => Self::ClassDef(StmtClassDef {
                 name: name.as_str(),
-                arguments: arguments.as_ref().map(Into::into),
+                arguments: arguments.as_ref().map(Into::into).unwrap_or_default(),
                 body: body.iter().map(Into::into).collect(),
                 decorator_list: decorator_list.iter().map(Into::into).collect(),
                 type_params: type_params.as_ref().map(Into::into),