[Bug] The variable assignment component used in the knowledge base workflow cannot refer to the parameters of the previous node.

### Contact Information

tangbowen

### MaxKB Version

2.4.1

### Problem Description

知识库工作流使用变量赋值组件不能引用前面节点的参数

### Steps to Reproduce

 变量赋值组件中变量为空

### The expected correct result

_No response_

### Related log output

```shell

```

### Additional Information

_No response_