antd的Select组件设置允许不选中任何项

可以设置`allowClear`属性为`true`，这样就可以在Select组件中显示一个清除按钮，点击清除按钮后就可以将选中的项清除掉，从而不选中任何项。示例代码如下：

import { Select } from 'antd';

const { Option } = Select;

function handleChange(value) {
  console.log(`selected ${value}`);
}

function handleBlur() {
  console.log('blur');
}

function handleFocus() {
  console.log('focus');
}

<Select
  showSearch
  style={{ width: 200 }}
  placeholder="Select a person"
  optionFilterProp="children"
  onChange={handleChange}
  onFocus={handleFocus}
  onBlur={handleBlur}
  filterOption={(input, option) =>
    option.children.toLowerCase().indexOf(input.toLowerCase()) >= 0
  }
  allowClear //设置允许清除
>
  <Option value="jack">Jack</Option>
  <Option value="lucy">Lucy</Option>
  <Option value="tom">Tom</Option>
</Select>