fix: model reducer return type

[luhc228]

解决使用 immer 更改 state 时 reducer 方法返回值错误的问题。如果使用 immer 更改 state，不需要返回任何值，比如：

const model = {
  state: {
    count: 1
  },
  reducers: {
     increment: (prevState) => {
      prevState.count++;
     }
  }
}

[codecov-commenter]

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