fix: dataset segment not exist return agent response (#1329)

api/core/agent/agent/multi_dataset_router_agent.py

                 if isinstance(tool_inputs, dict) and 'query' in tool_inputs:
                     tool_inputs['query'] = kwargs['input']
                     agent_decision.tool_input = tool_inputs
+            else:
+                agent_decision.return_values['output'] = ''
             return agent_decision
         except Exception as e:
             new_exception = self.model_instance.handle_exceptions(e)

api/core/agent/agent/structed_multi_dataset_router_agent.py

                 if isinstance(tool_inputs, dict) and 'query' in tool_inputs:
                     tool_inputs['query'] = kwargs['input']
                     agent_decision.tool_input = tool_inputs
+            else:
+                agent_decision.return_values['output'] = ''
             return agent_decision
         except OutputParserException:
             return AgentFinish({"output": "I'm sorry, the answer of model is invalid, "