Fix token_usage None issue in ChatOpenAI with local Chatglm2-6B (#14493)

When using local Chatglm2-6B by changing OPENAI_BASE_URL to localhost,
the token_usage in ChatOpenAI becomes None. This leads to an
AttributeError when trying to access token_usage.items().

This commit adds a check to ensure token_usage is not None before
accessing its items. This change prevents the AttributeError and allows
ChatOpenAI to work seamlessly with a local Chatglm2-6B model, aligning
with the way it operates with the OpenAI API.

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

libs/community/langchain_community/chat_models/openai.py

@@ -367,11 +367,12 @@ class ChatOpenAI(BaseChatModel):
                 # Happens in streaming
                 continue
             token_usage = output["token_usage"]
-            for k, v in token_usage.items():
-                if k in overall_token_usage:
-                    overall_token_usage[k] += v
-                else:
-                    overall_token_usage[k] = v
+            if token_usage is not None:
+                for k, v in token_usage.items():
+                    if k in overall_token_usage:
+                        overall_token_usage[k] += v
+                    else:
+                        overall_token_usage[k] = v
             if system_fingerprint is None:
                 system_fingerprint = output.get("system_fingerprint")
         combined = {"token_usage": overall_token_usage, "model_name": self.model_name}