community: delta in openai choice can be None, creates handler for that (#25954)

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- [X ] **PR title**

- [X ] **PR message**: 

     **Description:** adds a handler for when delta choice is None

     **Issue:** Fixes #25951
     **Dependencies:** Not applicable


- [ X] **Add tests and docs**: Not applicable

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from the root of the package(s) you've modified. See contribution
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Co-authored-by: Grande <Tom.Daniel.Grande@statsbygg.no>
Co-authored-by: Erick Friis <erick@langchain.dev>

libs/community/langchain_community/chat_models/openai.py

@@ -403,6 +403,8 @@ class ChatOpenAI(BaseChatModel):
             if len(chunk["choices"]) == 0:
                 continue
             choice = chunk["choices"][0]
+            if choice["delta"] is None:
+                continue
             chunk = _convert_delta_to_message_chunk(
                 choice["delta"], default_chunk_class
             )
@@ -495,6 +497,8 @@ class ChatOpenAI(BaseChatModel):
             if len(chunk["choices"]) == 0:
                 continue
             choice = chunk["choices"][0]
+            if choice["delta"] is None:
+                continue
             chunk = _convert_delta_to_message_chunk(
                 choice["delta"], default_chunk_class
             )