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/langchain_core/messages/base.py

@@ -71,7 +71,17 @@ class BaseMessageChunk(BaseMessage):
     def _merge_kwargs_dict(
         self, left: Dict[str, Any], right: Dict[str, Any]
     ) -> Dict[str, Any]:
-        """Merge additional_kwargs from another BaseMessageChunk into this one."""
+        """Merge additional_kwargs from another BaseMessageChunk into this one,
+        handling specific scenarios where a key exists in both dictionaries
+        but has a value of None in 'left'. In such cases, the method uses the
+        value from 'right' for that key in the merged dictionary.
+        Example:
+        If left = {"function_call": {"arguments": None}} and
+        right = {"function_call": {"arguments": "{\n"}}
+        then, after merging, for the key "function_call",
+        the value from 'right' is used,
+        resulting in merged = {"function_call": {"arguments": "{\n"}}.
+        """
         merged = left.copy()
         for k, v in right.items():
             if k not in merged:

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"}}
         )