[Bug] Step graph and "connectNulls = false" connects where it should not

### Version

5.5.1

### Link to Minimal Reproduction

https://echarts.apache.org/examples/en/editor.html?c=line-step&version=5.5.1

### Steps to Reproduce

Use above example and add null values in the middle of the dataset.

### Current Behavior

Lines are painted that are not allowed to be painted. I have marked these in red here:


### Expected Behavior

Marked area should not be there.

### Environment

```markdown
- OS:
- Browser:
- Framework:
```

### Any additional comments?

_No response_