Fix lambdas without params. in Python.

[bm-w]

A lambda without parameters does not have a syntax for the parameters, unlike a function without parameters (which does, containing just the ( and ) characters). The stanza for callables did not optionally match @params, causing it to miss lambdas without parameters entirely, which would then cause an error in a later stanza:

0: Error executing statement edge @lam.output -> @lam.call at (984, 3)
    src/stack-graphs.tsg:984:3:
    984 |   edge @lam.output -> @lam.call
        |   ^
    in stanza
    src/stack-graphs.tsg:975:1:
    975 | (lambda
        | ^
    matching (lambda) node
    test/lambdas.py:6:7:
    6 | foo = lambda: print(y)
        |       ^
1: Evaluating edge sink
2: Undefined scoped variable [syntax node lambda (6, 7)].call

The fragments of the parsed Tree-sitter tree for such lambda without and with parameters is:

(lambda [6, 7] - [6, 23])
  body: …)
(lambda [9, 7] - [9, 32])
  parameters: (lambda parameters …)
  body: …)

As of v0.23.5 of the Python grammar, lambda indeed declares an optional parameters field.

Making the @params capture optional and only emitting the edges if it’s there fixes the issue.

[Copilot]

Pull Request Overview

This PR fixes an issue with lambdas without parameters by making the capture of @params optional, ensuring that call edges are only emitted if parameters are present.

• Added a test case for lambdas without parameters.
• Verified lambdas with parameters continue to work as expected.

Files not reviewed (1)

• languages/tree-sitter-stack-graphs-python/src/stack-graphs.tsg: Language not supported