In graph，when the node link to it self ，the line is invisible almost

### Version
4.9.0

### Steps to reproduce
```js
option = {
title: {
text: 'Graph 简单示例'
},
tooltip: {},
animationDurationUpdate: 1500,
animationEasingUpdate: 'quinticInOut',
series: [
{
type: 'graph',
layout: 'none',
symbolSize: 10,
roam: true,
label: {
show: true
},
edgeSymbol: ['circle', 'arrow'],
edgeSymbolSize: [4, 10],
edgeLabel: {
fontSize: 20
},
data: [{
name: '节点1',
x: 300,
y: 300
}, {
name: '节点2',
x: 800,
y: 300
}],
// links: [],
links: [{
source: 0,
target: 1,
symbolSize: [5, 20],
label: {
show: true
},
lineStyle: {
curveness: 0.2
}
}, {
source: '节点2',
target: '节点1',
label: {
show: true
},
lineStyle: {
curveness: 0.2
}
}, {
source: '节点1',
target: '节点1',
// label: {
// show: true
// },
lineStyle: {
curveness: 0.2
}
}, ],
lineStyle: {
opacity: 0.9,
width: 2,
curveness: 0
}
}
]
};
```
### What is expected?
the link invisible almost

### What is actually happening?
the link obvious