fix: fix unstable function call response arguments missing (#2872)

api/core/model_runtime/model_providers/openai/llm/llm.py

                 if assistant_message_function_call:
                     # start of stream function call
                     delta_assistant_message_function_call_storage = assistant_message_function_call
+                    if delta_assistant_message_function_call_storage.arguments is None:
+                        delta_assistant_message_function_call_storage.arguments = ''
                     if not has_finish_reason:
                         continue