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_codegen/src/generator.rs

@@ -878,9 +878,11 @@ impl<'a> Generator<'a> {
                 range: _,
             }) => {
                 group_if!(precedence::LAMBDA, {
-                    let npos = parameters.args.len() + parameters.posonlyargs.len();
-                    self.p(if npos > 0 { "lambda " } else { "lambda" });
-                    self.unparse_parameters(parameters);
+                    self.p("lambda");
+                    if let Some(parameters) = parameters {
+                        self.p(" ");
+                        self.unparse_parameters(parameters);
+                    }
                     self.p(": ");
                     self.unparse_expr(body, precedence::LAMBDA);
                 });