core[patch]: Fix NPE in function_calling._get_python_function_required_args (#21863)

Example error message:
line 206, in _get_python_function_required_args
    if is_function_type and required[0] == "self":
                            ~~~~~~~~^^^
IndexError: list index out of range

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---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/core/langchain_core/utils/function_calling.py

@@ -203,9 +203,9 @@ def _get_python_function_required_args(function: Callable) -> List[str]:
 
     is_function_type = isinstance(function, FunctionType)
     is_method_type = isinstance(function, MethodType)
-    if is_function_type and required[0] == "self":
+    if required and is_function_type and required[0] == "self":
         required = required[1:]
-    elif is_method_type and required[0] == "cls":
+    elif required and is_method_type and required[0] == "cls":
         required = required[1:]
     return required
 

libs/core/tests/unit_tests/utils/test_function_calling.py

@@ -146,6 +146,16 @@ def test_function_optional_param() -> None:
     assert set(req) == {"b"}
 
 
+def test_function_no_params() -> None:
+    def nullary_function() -> None:
+        """nullary function"""
+        pass
+
+    func = convert_to_openai_function(nullary_function)
+    req = func["parameters"]["required"]
+    assert not req
+
+
 class FakeCall(BaseModel):
     data: str