[nit] Add default value for ChatOpenAI client (#7939)

Micro convenience PR to avoid warning regarding missing `client`
parameter. It is always set during initialization.

@baskaryan

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/langchain/langchain/chat_models/openai.py

@@ -190,7 +190,7 @@ class ChatOpenAI(BaseChatModel):
     def lc_serializable(self) -> bool:
         return True
 
-    client: Any  #: :meta private:
+    client: Any = None  #: :meta private:
     model_name: str = Field(default="gpt-3.5-turbo", alias="model")
     """Model name to use."""
     temperature: float = 0.7