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_formatter/src/other/parameters.rs

@@ -7,8 +7,8 @@ use ruff_python_trivia::{SimpleToken, SimpleTokenKind, SimpleTokenizer};
 use ruff_text_size::{TextRange, TextSize};
 
 use crate::comments::{
-    dangling_open_parenthesis_comments, leading_comments, leading_node_comments, trailing_comments,
-    CommentLinePosition, SourceComment,
+    dangling_comments, dangling_open_parenthesis_comments, leading_comments, leading_node_comments,
+    trailing_comments, CommentLinePosition, SourceComment,
 };
 use crate::context::{NodeLevel, WithNodeLevel};
 use crate::expression::parentheses::empty_parenthesized;
@@ -242,7 +242,7 @@ impl FormatNodeRule<Parameters> for FormatParameters {
             + usize::from(kwarg.is_some());
 
         if self.parentheses == ParametersParentheses::Never {
-            write!(f, [group(&format_inner)])
+            write!(f, [group(&format_inner), dangling_comments(dangling)])
         } else if num_parameters == 0 {
             // No parameters, format any dangling comments between `()`
             write!(f, [empty_parenthesized("(", dangling, ")")])