fix: ignore empty string reasoning_content in _extract_reasoning_from_additional_kwargs

Some providers (e.g. Groq) include "reasoning_content": "" in
additional_kwargs even when no reasoning was produced. The previous
check `reasoning_content is not None` treated an empty string as valid,
inserting an empty ReasoningContentBlock into the message content list.
Change to a truthy check so empty strings are skipped.

Fixes #36194

Co-Authored-By: hlwz5735 <12751178+hlwz5735@users.noreply.github.com>

Author: VANDRANKI

libs/core/langchain_core/messages/base.py

@@ -38,7 +38,7 @@ def _extract_reasoning_from_additional_kwargs(
     additional_kwargs = getattr(message, "additional_kwargs", {})
 
     reasoning_content = additional_kwargs.get("reasoning_content")
-    if reasoning_content is not None and isinstance(reasoning_content, str):
+    if reasoning_content and isinstance(reasoning_content, str):
         return {"type": "reasoning", "reasoning": reasoning_content}
 
     return None