[Bug] No markLine drawn if one of the markLines is empty

### Version

6.1

### Link to Minimal Reproduction

https://jsfiddle.net/Lvop3jc7/

### Steps to Reproduce

1. Add a series with markLine and then inside the data of the markLine have one empty object
` markLine: {
data: [
{
"yAxis": 1
},
{},
{
"yAxis": 3
},
]
}`

### Current Behavior

No markLine is rendered.
There is a error Uncaught TypeError: Cannot read properties of undefined (reading 'coord')

### Expected Behavior

The empty markLine is skipped in rendering, but the other marklines are rendered. No error.

### Environment

```markdown
- OS: Windows 11
- Browser: Chrome
- Framework:
```

### Any additional comments?

_No response_

Start with the linked JSFiddle reproduction and trace the markLine rendering path for the data array containing an empty object. Confirm that the empty entry is skipped while the yAxis 1 and yAxis 3 lines still render without the reported TypeError; add or update coverage if the relevant test location is found.

Written by the indexing model from the issue text.