replace message content type list to string when file_objs is empty .. (#2745)

api/core/memory/token_buffer_memory.py

                     files, message.app_model_config
                 )
 
-                prompt_message_contents = [TextPromptMessageContent(data=message.query)]
-                for file_obj in file_objs:
-                    prompt_message_contents.append(file_obj.prompt_message_content)
-
-                prompt_messages.append(UserPromptMessage(content=prompt_message_contents))
+                if not file_objs:
+                    prompt_messages.append(UserPromptMessage(content=message.query))
+                else:
+                    prompt_message_contents = [TextPromptMessageContent(data=message.query)]
+                    for file_obj in file_objs:
+                        prompt_message_contents.append(file_obj.prompt_message_content)
+
+                    prompt_messages.append(UserPromptMessage(content=prompt_message_contents))
             else:
                 prompt_messages.append(UserPromptMessage(content=message.query))