fix: prevent adding an empty string to the result queue in AsyncIteratorCallbackHandler (#7180)

- Description: Modify the code for
AsyncIteratorCallbackHandler.on_llm_new_token to ensure that it does not
add an empty string to the result queue.
- Tag maintainer: @agola11

When using AsyncIteratorCallbackHandler with OpenAIFunctionsAgent, if
the LLM response function_call instead of direct answer, the
AsyncIteratorCallbackHandler.on_llm_new_token would be called with empty
string.
see also: langchain.chat_models.openai.ChatOpenAI._generate

An alternative solution is to modify the
langchain.chat_models.openai.ChatOpenAI._generate and do not call the
run_manager.on_llm_new_token when the token is empty string.
I am not sure which solution is better.

@hwchase17

---------

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

langchain/callbacks/streaming_aiter.py

@@ -31,6 +31,7 @@ class AsyncIteratorCallbackHandler(AsyncCallbackHandler):
         self.done.clear()
 
     async def on_llm_new_token(self, token: str, **kwargs: Any) -> None:
+        if token is not None and token != "":
             self.queue.put_nowait(token)
 
     async def on_llm_end(self, response: LLMResult, **kwargs: Any) -> None: