feat(tool_call): changed return type of tool_call

Update the tool call response format so that all non-first chunks in a tool call have type: null.

python/sglang/srt/entrypoints/openai/serving_chat.py

@@ -1359,7 +1359,8 @@ class OpenAIServingChat(OpenAIServingBase):
                     arguments=call_item.parameters,
                 ),
             )
-
+            if tool_call_id is None:
+                tool_call.type = None
             choice_data = ChatCompletionResponseStreamChoice(
                 index=index,
                 delta=DeltaMessage(tool_calls=[tool_call]),