[Bug] stack bar with only encode.y cannot be stacked

### Version

5.6.0

### Link to Minimal Reproduction

[demo](https://echarts.apache.org/examples/zh/editor.html?c=bar-simple&code=PYBwLglsB2AEC8sDeAoWsAeBBDEDOAXMmurGAJ4gCmRA5AMYCGYVA5sAE7m0A0J6AegGwAJs0ZEA2rVoBdEgF8-6cjnxFUpMpRqxaAN0YAbAK5Vai5WWDAjkEBv6w8AC2AB3ImA5mr6bxCsrFQcdIy4eBboSiR4IRBUhLCSTppasEKi4lIAzAAMsn6kVND0wCK6SLDkUgWwMenOYIz0ANZ0eM1tvE7-OnQARowcPY1GjANURo6N6K4eXj5UvfVODaRpWplizVIAjAVF6CVlFRrV-7L1R00t7Xqdd6PpFNSDw89a45PTxLPObk8ZCWKwUaxIhRIO0YcTAUlSKzwwBMHHouhS_0k-R4sAO8ka-NIYPQkIUAG4gA)

### Steps to Reproduce

If there are only values in the dataset.
Using `encode.y` and `stack` bar cannot stack the bars
I tried to add a dummy `data: [ '' ]` to `xAxis`, still doesn't work

### Current Behavior

bars are not stacked

### Expected Behavior

bars should be stacked

### Environment

```markdown
- OS:
- Browser:
- Framework:
```

### Any additional comments?

_No response_