区域填充颜色重叠异常

### Version
4.8.0

### Steps to reproduce
```js
option = {
xAxis: {
type: 'category',
boundaryGap: false,
data: ['周一','周二','周三','周四','周五','周六','周日']
},
yAxis: {
type: 'value'
},
series: [{
name: '成交',
type: 'line',
smooth: true,
connectNulls: true,
stack: "stack",
areaStyle: {},
data: [10, "-", "-", 710, "-", "-", 54]
},
{
name: '预购',
type: 'line',
smooth: true,
connectNulls: true,
stack: "stack",
areaStyle: {},
data: [10, "-", "-", 710, "-", "-", 54]
}]
};
```

### What is expected?
重叠后应该只有两种颜色，而非三种

### What is actually happening?
重叠后出现三种颜色