svg模式下,折线图在数据都为0的情况下,设置shadowOffset或shadowBlur会导致折线不显示

### Version
5.1.2

### Steps to reproduce
1.renderer设置为svg
2.正常设置折线图
3.折线图数据value值都为0
4.series中给lineStyle设置shadowoffset或shadowblue

A simple demo

```js
option = {
xAxis: {
type: 'category',
data: ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
},
yAxis: {
type: 'value'
},
series: [{
data: [0, 0, 0, 0, 0, 0, 0],
type: 'line',
stack: true,
color: '#7f64ff',
symbol: 'none',
smooth: true,
lineStyle: {
shadowColor: 'red',
shadowBlur: 5,
shadowOffsetX: 0,
shadowOffsetY: 0
}
}]
};
```

### What is expected?
线条正常在底部显示

### What is actually happening?
线条不显示

Start by running the provided ECharts option with the SVG renderer, using all-zero line data and the shown shadow settings. Compare the result with the expected chart and trace the SVG line rendering behavior; done means the line is visible at the bottom under these conditions.

Written by the indexing model from the issue text.