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/resources/test/fixtures/ruff/expression/lambda.py

@@ -56,12 +56,16 @@ lambda x: lambda y: lambda z: (
 z)  # Trailing
 # Trailing
 
-
 a = (
     lambda  # Dangling
            : 1
 )
 
+a = (
+    lambda x # Dangling
+    , y: 1
+)
+
 # Regression test: lambda empty arguments ranges were too long, leading to unstable
 # formatting
 (lambda:(#
@@ -93,3 +97,24 @@ lambda a, *args, b, **kwds,: 0
 lambda a, *, b, **kwds,: 0
 lambda a, /: a
 lambda a, /, c: a
+
+# Dangling comments without parameters.
+(
+    lambda
+    : # 3
+     None
+)
+
+(
+    lambda
+    # 3
+    : None
+)
+
+(
+    lambda  # 1
+    # 2
+    : # 3
+    # 4
+    None # 5
+)