Relax the action input check for actions that require no input (#6357)

When the tool requires no input, the LLM often gives something like
this:
```json
{
    "action": "just_do_it"
}
```
I have attempted to enhance the prompt, but it doesn't appear to be
functioning effectively. Therefore, I believe we should consider easing
the check a little bit.



Signed-off-by: Xiaochao Dong (@damnever) <the.xcdong@gmail.com>

langchain/agents/chat/output_parser.py

@@ -17,13 +17,15 @@ class ChatOutputParser(AgentOutputParser):
         try:
             action = text.split("```")[1]
             response = json.loads(action.strip())
-            includes_action = "action" in response and "action_input" in response
+            includes_action = "action" in response
             if includes_answer and includes_action:
                 raise OutputParserException(
                     "Parsing LLM output produced a final answer "
                     f"and a parse-able action: {text}"
                 )
-            return AgentAction(response["action"], response["action_input"], text)
+            return AgentAction(
+                response["action"], response.get("action_input", {}), text
+            )
 
         except Exception:
             if not includes_answer: