partners: (langchain-deepseek) fix deepseek-r1 always returns an empty reasoning_content when reasoning (#31065)

## Description
deepseek-r1 always returns an empty string `reasoning_content` to the
first chunk when thinking, and sets `reasoning_content` to None when
thinking is over, to determine when to switch to normal output.

Therefore, whether the reasoning_content field exists should be judged
as None.

## Demo
deepseek-r1 reasoning output: 

```
{'delta': {'content': None, 'function_call': None, 'refusal': None, 'role': 'assistant', 'tool_calls': None, 'reasoning_content': ''}, 'finish_reason': None, 'index': 0, 'logprobs': None}
{'delta': {'content': None, 'function_call': None, 'refusal': None, 'role': None, 'tool_calls': None, 'reasoning_content': '好的'}, 'finish_reason': None, 'index': 0, 'logprobs': None}
{'delta': {'content': None, 'function_call': None, 'refusal': None, 'role': None, 'tool_calls': None, 'reasoning_content': '，'}, 'finish_reason': None, 'index': 0, 'logprobs': None}
{'delta': {'content': None, 'function_call': None, 'refusal': None, 'role': None, 'tool_calls': None, 'reasoning_content': '用户'}, 'finish_reason': None, 'index': 0, 'logprobs': None}
...
```

deepseek-r1 first normal output
```
...
{'delta': {'content': ' main', 'function_call': None, 'refusal': None, 'role': None, 'tool_calls': None, 'reasoning_content': None}, 'finish_reason': None, 'index': 0, 'logprobs': None}
{'delta': {'content': '\n\nimport', 'function_call': None, 'refusal': None, 'role': None, 'tool_calls': None, 'reasoning_content': None}, 'finish_reason': None, 'index': 0, 'logprobs': None}
...
```

---------

Co-authored-by: ccurme <chester.curme@gmail.com>

Author: reatang

libs/partners/deepseek/langchain_deepseek/chat_models.py

@@ -255,12 +255,14 @@ def _convert_chunk_to_generation_chunk(
         if (choices := chunk.get("choices")) and generation_chunk:
             top = choices[0]
             if isinstance(generation_chunk.message, AIMessageChunk):
-                if reasoning_content := top.get("delta", {}).get("reasoning_content"):
+                if (
+                    reasoning_content := top.get("delta", {}).get("reasoning_content")
+                ) is not None:
                     generation_chunk.message.additional_kwargs["reasoning_content"] = (
                         reasoning_content
                     )
                 # Handle use via OpenRouter
-                elif reasoning := top.get("delta", {}).get("reasoning"):
+                elif (reasoning := top.get("delta", {}).get("reasoning")) is not None:
                     generation_chunk.message.additional_kwargs["reasoning_content"] = (
                         reasoning
                     )