mergeLonghand breaks border-color

`mergeLonghand` breaks CSS in `cssnano@4.0.2`.

Input:

```css
.a {
    border: solid transparent;
    border-width: 0 8px 16px;
    border-bottom-color: #eee;
}
```

Output:

```css
.a {
  border: solid transparent;
  border-color: currentColor currentColor #eee;
  border-width: 0 8px 16px;
}
```

`currentColor` is black by default, so instead of transparent, we have black.