[Bugfix][Tool Call] Add null system prompt to support tool system prompt (#15092)

Signed-off-by: Muqi Li <muqi1029@gmail.com>

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

@@ -293,6 +293,10 @@ class OpenAIServingChat(OpenAIServingBase):
             )
             messages = request.messages
             messages = [msg.model_dump() for msg in messages]
+
+            if messages[0]["role"] != "system":
+                # insert an empty system prompt to help render tool system prompt
+                messages.insert(0, {"role": "system", "content": ""})
             if request.tools:
                 messages[0]["tools"] = [tool.model_dump() for tool in request.tools]
             real_input = encode_messages(messages, thinking_mode=thinking_mode)