[Bug] visualMap.pieces has only one obj and obj has only one gt TypeError

### Version

4.9.0~5.4.3

### Link to Minimal Reproduction

pieces: [{gt: 0, color: 'red'}]

### Steps to Reproduce

```js
const hours = ['17时', '18时', '19时', '20时', '21时', '22时', '24时'];
const values = [4, 4, 4, 10, 10, 10, 10];
const colors = [
{
gt: 0,
// lte: 7,
color: 'blue'
}
];

option = {
title: {
text: 'xxx',
},
tooltip: {
trigger: 'axis',
axisPointer: {
type: 'cross'
}
},
xAxis: {
type: 'category',
boundaryGap: false,
// prettier-ignore
data: hours
},
yAxis: {
type: 'value',
axisLabel: {
formatter: '{value} kwh'
},
axisPointer: {
snap: true
}
},
visualMap: {
show: false,
dimension: 0,
pieces: colors
},
series: [
{
name: '用电量',
type: 'line',
data: values,
}
]
};
```

### Current Behavior

TypeError: Cannot read properties of undefined (reading 'coord')
at getVisualGradient (LineView.js:228:1)
at ExtendedClass.render (LineView.js:469:1)
at Task.progress (Chart.js:281:1)
at doProgress (task.js:217:1)
at taskProto.perform (task.js:157:1)
at eval (echarts.js:1474:1)
at ExtendedClass.eval (Global.js:517:1)
at Array.forEach ()
at each (util.js:300:1)
at ExtendedClass.eachSeries (Global.js:514:1)

### Expected Behavior

Running successed!

### Environment

```markdown
- OS: Windows 10
- Browser: Edge
- Framework: Vue.js
```

### Any additional comments?

When pieces has only one obj and obj only has gt and color prop, then run with TypeError like 'TypeError: Cannot read properties of undefined (reading 'coord')'. However, it works fine when obj has gt and lte.
pieces: [{
gt: 0,
// lte: 7,
color: 'blue'
}]