[Bug] `null` values in line chart with visual map behaves randomly

### Version

5.6.0

### Link to Minimal Reproduction

https://codepen.io/cracksalad/pen/Pwwpzeq

### Steps to Reproduce

1. Setup a line chart with visual map
2. Add some `null` values in the dimension the visual map operates on.

### Current Behavior

The result is somewhat random: In the example I linked via CodePen there are two datasets. The first works totally fine and marks some data points with red triangles while the other dataset is not marked at all, although it has data points to be marked as well (see dimension 2). While searching for the cause of this behavior I had examples having multiple datasets, none of them working as well as examples having multiple datasets and all of them worked fine. The one constant observation is, that it affects a whole dataset or not. There is no "some data points in a dataset are affected, others are not".
As soon as one replaces the `null` values in dimension 2 with say empty strings, everything works fine.

### Expected Behavior

Both datasets in the CodePen example should have data points marked with red triangles.

### Environment

```markdown
- OS: Ubuntu 24.04 x64, Windows 10 x64
- Browser: Firefox 137, Chromium 135
- Framework: none, ngx-echarts (Angular)
```

### Any additional comments?

After many hours of trial and error I found this note about "empty value" [here in the docs](https://echarts.apache.org/en/option.html#series-line.data) which pointed me in the right direction. Although I am not even using `series.data`.
So I think this behavior should be documented more clearly if expected or fixed if unexpected of course.