Make Parameters an optional field on ExprLambda (#6669)

## Summary

If a lambda doesn't contain any parameters, or any parameter _tokens_
(like `*`), we can use `None` for the parameters. This feels like a
better representation to me, since, e.g., what should the `TextRange` be
for a non-existent set of parameters? It also allows us to remove
several sites where we check if the `Parameters` is empty by seeing if
it contains any arguments, so semantically, we're already trying to
detect and model around this elsewhere.

Changing this also fixes a number of issues with dangling comments in
parameter-less lambdas, since those comments are now automatically
marked as dangling on the lambda. (As-is, we were also doing something
not-great whereby the lambda was responsible for formatting dangling
comments on the parameters, which has been removed.)

Closes https://github.com/astral-sh/ruff/issues/6646.

Closes https://github.com/astral-sh/ruff/issues/6647.

## Test Plan

`cargo test`

crates/ruff_python_ast/src/comparable.rs

@@ -542,7 +542,7 @@ pub struct ExprUnaryOp<'a> {
 
 #[derive(Debug, PartialEq, Eq, Hash)]
 pub struct ExprLambda<'a> {
-    parameters: ComparableParameters<'a>,
+    parameters: Option<ComparableParameters<'a>>,
     body: Box<ComparableExpr<'a>>,
 }
 
@@ -773,7 +773,7 @@ impl<'a> From<&'a ast::Expr> for ComparableExpr<'a> {
                 body,
                 range: _,
             }) => Self::Lambda(ExprLambda {
-                parameters: (parameters.as_ref()).into(),
+                parameters: parameters.as_ref().map(Into::into),
                 body: body.into(),
             }),
             ast::Expr::IfExp(ast::ExprIfExp {

crates/ruff_python_ast/src/node.rs

@@ -2038,7 +2038,9 @@ impl AstNode for ast::ExprLambda {
             range: _,
         } = self;
 
-        visitor.visit_parameters(parameters);
+        if let Some(parameters) = parameters {
+            visitor.visit_parameters(parameters);
+        }
         visitor.visit_expr(body);
     }
 }

crates/ruff_python_ast/src/nodes.rs

@@ -632,7 +632,7 @@ impl From<ExprUnaryOp> for Expr {
 #[derive(Clone, Debug, PartialEq)]
 pub struct ExprLambda {
     pub range: TextRange,
-    pub parameters: Box<Parameters>,
+    pub parameters: Option<Box<Parameters>>,
     pub body: Box<Expr>,
 }
 

crates/ruff_python_ast/src/visitor.rs

@@ -354,7 +354,9 @@ pub fn walk_expr<'a, V: Visitor<'a> + ?Sized>(visitor: &mut V, expr: &'a Expr) {
             body,
             range: _,
         }) => {
-            visitor.visit_parameters(parameters);
+            if let Some(parameters) = parameters {
+                visitor.visit_parameters(parameters);
+            }
             visitor.visit_expr(body);
         }
         Expr::IfExp(ast::ExprIfExp {