Label position does not reasonable when both positive and negative value stacked together.

### Version
5.1.2

### Steps to reproduce
```js

var option = {
xAxis: {
type: 'category',
data: ['a', 'b', 'c']
},
yAxis: {
inverse: true
},
legend: {
},
series: [{
type: 'bar',
stack: 'all',
data: [
10, 20, 30
],
label: {
show: true,
fontSize: 30
},
name: 'positive'
}, {
type: 'bar',
stack: 'all',
label: {
show: true,
fontSize: 30
},
data: [
-10, 0, -15
],
name: 'negative'
}]
};
```

Key points of reproduce:
+ `yAxis.inverse: true`
+ The two series are stacked and the one has positive values and the other has negative values.

When value is 0, we count it as "positive stack". But we may make it more wise that determine it by other values in the same series. (If other values are negative, then 0 should belongs to "negative stack").

Test case:

Start with test/bar-stack.html and reproduce the stacked bar case using an inverse yAxis, positive values, negative values, and a zero value. Check how the test currently renders labels for the zero value; done means the label position is reasonable when the zero belongs with the negative stack, without regressing positive-stack labels.

Written by the indexing model from the issue text.