Fix Azure Openai function calling in streaming mode (#13768)

- **Description**: This PR addresses an issue with the OpenAI API
streaming response, where initially the key (arguments) is provided but
the value is None. Subsequently, it updates with {"arguments": "{\n"},
leading to a type inconsistency that causes an exception. The specific
error encountered is ValueError: additional_kwargs["arguments"] already
exists in this message, but with a different type. This change aims to
resolve this inconsistency and ensure smooth API interactions.
- **Issue**: None.
- **Dependencies**: None.
- **Tag maintainer**: @eyurtsev

This is an updated version of #13229 based on the refactored code.
Credit goes to @superken01.

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

libs/core/tests/unit_tests/test_messages.py

@@ -42,6 +42,9 @@ def test_message_chunks() -> None:
         AIMessageChunk(
             content="", additional_kwargs={"function_call": {"name": "web_search"}}
         )
+        + AIMessageChunk(
+            content="", additional_kwargs={"function_call": {"arguments": None}}
+        )
         + AIMessageChunk(
             content="", additional_kwargs={"function_call": {"arguments": "{\n"}}
         )