Bug Fix: AzureChatOpenAI streaming with function calls (#8300)

- Description: During streaming, the first chunk may only contain the
name of an OpenAI function and not any arguments. In this case, the
current code presumes there is a streaming response and tries to append
to it, but gets a KeyError. This fixes that case by checking if the
arguments key exists, and if not, creates a new entry instead of
appending.
  - Issue: Related to #6462

Sample Code:
```python
llm = AzureChatOpenAI(
    deployment_name=deployment_name,
    model_name=model_name,
    streaming=True
)

tools = [PythonREPLTool()]
callbacks = [StreamingStdOutCallbackHandler()]

agent = initialize_agent(
    tools=tools,
    llm=llm,
    agent=AgentType.OPENAI_FUNCTIONS,
    callbacks=callbacks
)

agent('Run some python code to test your interpreter')
```

Previous Result:
```
File ...langchain/chat_models/openai.py:344, in ChatOpenAI._generate(self, messages, stop, run_manager, **kwargs)
    342         function_call = _function_call
    343     else:
--> 344         function_call["arguments"] += _function_call["arguments"]
    345 if run_manager:
    346     run_manager.on_llm_new_token(token)

KeyError: 'arguments'
```

New Result:
```python
{'input': 'Run some python code to test your interpreter',
 'output': "The Python code `print('Hello, World!')` has been executed successfully, and the output `Hello, World!` has been printed."}
```

Co-authored-by: jswe <jswe@polencapital.com>

libs/langchain/langchain/chat_models/openai.py

@@ -340,8 +340,10 @@ class ChatOpenAI(BaseChatModel):
                     if _function_call:
                         if function_call is None:
                             function_call = _function_call
-                        else:
+                        elif "arguments" in function_call:
                             function_call["arguments"] += _function_call["arguments"]
+                        else:
+                            function_call["arguments"] = _function_call["arguments"]
                     if run_manager:
                         run_manager.on_llm_new_token(token)
             message = _convert_dict_to_message(
@@ -406,8 +408,10 @@ class ChatOpenAI(BaseChatModel):
                     if _function_call:
                         if function_call is None:
                             function_call = _function_call
-                        else:
+                        elif "arguments" in function_call:
                             function_call["arguments"] += _function_call["arguments"]
+                        else:
+                            function_call["arguments"] = _function_call["arguments"]
                     if run_manager:
                         await run_manager.on_llm_new_token(token)
             message = _convert_dict_to_message(