[Bug] Adding shadow makes markLine disappear if the line is horizontal or vertical

### Version

6.0.0

### Link to Minimal Reproduction

https://codepen.io/soundlake/pen/yyORLNg

### Steps to Reproduce

1. Create chart with `markLine`
2. Add data that can produce horizontal (no height) or vertical (no width) mark line
3. Add `shadow*` to the `lineStyle`

### Current Behavior

The line gets disappeared while symbols and label are shown.

### Expected Behavior

The line also be shown.

### Environment

```markdown
- OS: Windows 11
- Browser: Edge 142
- Framework: Plain JS
```

### Any additional comments?

It might be a `zrender` bug, but at least, the doc should warn.

This is a known issue for SVG, at least on [StackOverflow](https://stackoverflow.com/questions/47758565/adding-fedropshadow-to-a-vertical-line-in-svg-makes-it-disappear).