Invalid minify result of `background-position: center bottom 1px`

### Description

This CSS rule
```css
background-position: center bottom 1px;
```
is minified to
```css
background-position: 50% bottom 1px;
```
which is invalid.

![20240711_231247](https://github.com/parcel-bundler/lightningcss/assets/47248675/dae8f615-8e4b-4d90-9a40-e75c9a1aa42a)

### Expected result

`center` should be kept.

### Version

1.25.1