fix crash when using create_xml_agent with parameterless function as … (#26002)

When using `create_xml_agent` or `create_json_chat_agent` to create a
agent, and the function corresponding to the tool is a parameterless
function, the `XMLAgentOutputParser` or `JSONAgentOutputParser` will
parse the tool input into an empty string, `BaseTool` will parse it into
a positional argument.
So, the program will crash finally because we invoke a parameterless
function but with a positional argument.Specially, below code will raise
StopIteration in
[_parse_input](https://github.com/langchain-ai/langchain/blob/master/libs/core/langchain_core/tools/base.py#L419)
```python
from langchain import hub
from langchain.agents import AgentExecutor, create_json_chat_agent, create_xml_agent
from langchain_openai import ChatOpenAI

prompt = hub.pull("hwchase17/react-chat-json")

llm = ChatOpenAI()

# agent = create_xml_agent(llm, tools, prompt)
agent = create_json_chat_agent(llm, tools, prompt)
agent_executor = AgentExecutor(agent=agent, tools=tools, verbose=True)

agent_executor.invoke(......)
```

---------

Co-authored-by: Erick Friis <erick@langchain.dev>
Co-authored-by: Bagatur <22008038+baskaryan@users.noreply.github.com>
Co-authored-by: Chester Curme <chester.curme@gmail.com>

libs/core/langchain_core/tools/base.py

@@ -605,6 +605,9 @@ class ChildTool(BaseTool):
     def _to_args_and_kwargs(
         self, tool_input: Union[str, dict], tool_call_id: Optional[str]
     ) -> tuple[tuple, dict]:
+        if self.args_schema is not None and not get_fields(self.args_schema):
+            # StructuredTool with no args
+            return (), {}
         tool_input = self._parse_input(tool_input, tool_call_id)
         # For backwards compatibility, if run_input is a string,
         # pass as a positional argument.

libs/core/tests/unit_tests/test_tools.py

@@ -575,6 +575,19 @@ def test_structured_tool_from_function_with_run_manager() -> None:
     )
 
 
+def test_structured_tool_from_parameterless_function() -> None:
+    """Test parameterless function of structured tool."""
+
+    def foo() -> str:
+        """Docstring."""
+        return "invoke foo"
+
+    structured_tool = StructuredTool.from_function(foo)
+
+    assert structured_tool.run({}) == "invoke foo"
+    assert structured_tool.run("") == "invoke foo"
+
+
 def test_named_tool_decorator() -> None:
     """Test functionality when arguments are provided as input to decorator."""