Fix formatter crash on bare return in lambda

The return_stmt handler unconditionally accessed s.children[0], but a
bare `return` (without expression) has zero children. This caused an
IndexError when formatting lambdas like `func(): return`.

The grammar correctly defines return_stmt with an optional expression
(`return_stmt: "return" [expr]`), so the formatter now checks for
empty children before accessing the list.

Author: kuruk-mm

gdtoolkit/formatter/function_statement_to_str.py

@@ -12,7 +12,7 @@ def function_statement_to_str(statement: Tree) -> str:
         "func_var_stmt": lambda s: function_statement_to_str(s.children[0]),
         "const_stmt": lambda s: function_statement_to_str(s.children[0]),
         "expr_stmt": lambda s: expression_to_str(s.children[0]),  # TODO: standalone?
-        "return_stmt": lambda s: f"return {standalone_expression_to_str(s.children[0])}",
+        "return_stmt": lambda s: "return" if len(s.children) == 0 else f"return {standalone_expression_to_str(s.children[0])}",
         "break_stmt": _not_implemented,
         "breakpoint_stmt": lambda _: "breakpoint",
         "continue_stmt": _not_implemented,