fix merge None value and non None value error (#13703)

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libs/core/langchain_core/messages/base.py

@@ -76,6 +76,8 @@ class BaseMessageChunk(BaseMessage):
         for k, v in right.items():
             if k not in merged:
                 merged[k] = v
+            elif merged[k] is None and v:
+                merged[k] = v
             elif type(merged[k]) != type(v):
                 raise ValueError(
                     f'additional_kwargs["{k}"] already exists in this message,'