[Feature] [3.x] stroke of labels should be the same with legend when set to null. 标签描边颜色支持跟随图例

[da730]

What problem does this feature solve?

现在的label.style.stroke应该与.label.style.fill对齐，当设置为null时，描边颜色跟随图例。否则无法实现这种效果：

What does the proposed API look like?

label: {
style: {
fill: '#fff',
stroke: null
}
}