Update lmdeploy/serve/openai/api_server.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: windreamer

lmdeploy/serve/openai/api_server.py

@@ -414,11 +414,17 @@ async def chat_completions_v1(request: ChatCompletionRequest, raw_request: Reque
                     request.messages += format_section
                 else:
                     messages = request.messages
+                    appended_to_system = False
                     for msg in messages:
-                        if msg['role'] == 'system':
-                            msg['content'] += format_section
-                            break
-                    else:
+                        if msg.get('role') == 'system':
+                            content = msg.get('content')
+                            if content is None:
+                                content = ''
+                            if isinstance(content, str):
+                                msg['content'] = content + format_section
+                                appended_to_system = True
+                                break
+                    if not appended_to_system:
                         system_msg = {
                             'role': 'system',
                             'content': f'You must follow the specified response format.{format_section}'