[Bug] selected in legend cannot retrieve information

### Version

https://echarts.apache.org/en/js/vendors/echarts/dist/echarts.min.js

### Link to Minimal Reproduction

https://codesandbox.io/p/sandbox/basic-line-chart-forked-8frt7c?file=%2Findex.js%3A13%2C5

### Steps to Reproduce

open the console and you can see that the output information is {}

### Current Behavior

open the console and you can see that the output information is {}


### Expected Behavior

The selected in getOption can output the current information, such as the current status.

```
{
'Union Ads': true,
'Video Ads': true,
'Direct': true,
'Search Engine': true,
'Email': true
}
```

### Environment

```markdown
- OS: macOS Sonoma 14.1.1 (23B81)
- Browser: Chrome 134.0.6998.166
```

### Any additional comments?

_No response_