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 {