Y-axis label of a bar chart disappears if `realtimeSort` is true and there is just one pair of key-value format source in `dataset`

### Version
5.1.1

### Reproduction link
[https://codepen.io/zaakn/pen/KKWZPVm](https://codepen.io/zaakn/pen/KKWZPVm)

show the code just same with the codepen above

```javascript
var mychart = echarts.init(document.getElementById('mychart'));

var sources = {
bar: [{key: "BarKey1", value: 30}],
// if the foo array has been sorted [10, 20], the y-axis label issue will occur.
fooSorted: [{key: "FooKey1", value: 10}, {key: "FooKey2", value: 20}],
// if [20, 10], not occur
fooUnsorted: [{key: "FooKey2", value: 20}, {key: "FooKey1", value: 10}]
}

var option = {
dataset: { source: sources["fooSorted"] },
xAxis: { type: "value" },
yAxis: { type: "category"},
series: [
{ type: "bar", realtimeSort: true }
]
}

document.getElementById("switch").addEventListener("click", function(ev) {
const { dataset: ds } = option;
const { checked } = document.getElementById("sorted");
const wantedKey = !!checked ? "fooSorted" : "fooUnsorted";

ds.source = ds.source == sources[wantedKey] ? sources["bar"] : sources[wantedKey];

mychart.setOption(option);
})

mychart.setOption(option);
```

```html

Click Me
sorted data

### Steps to reproduce
1. `realtimeSort` is `true` in a bar `series`
2. use the key-value format data in `dataset`, like:
```json
[
{key: "FooKey1", value: 10},
{key: "FooKey2", value: 20}
]
```
3. the source data array is sorted. (ascending, whatever `yAxis.inverse` is)
4. set `realtimeSort` in `series` instead of `yAxis`, [ref from official example](https://echarts.apache.org/examples/en/editor.html?c=bar-race)
5. the issue will occur only when the dataset source has been changed.

### What is expected?
the y-axis label is shown.

### What is actually happening?
the y-axis label is hidden.

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