Responsive line-height with default parameters does not work

Hi. I'm using postcss-responsive-type@1.0.0.
I just wanted to use responsive line-height.

```css
html {
line-height: responsive;
}
```

Unfortunately it generates an error:

```
postcss-responsive-type: $FILE_PATH/default.css:5:1: this combination of units is not supported
```

and invalid css:

```css
html {
line-height: calc(1.2em + 0.6000000000000001 * ((100vw - undefined) / NaN));
}

@media screen and (min-width: 1280px) {
html {
line-height: 1.8em;
}
}

@media screen and (max-width: 420px) {
html {
line-height: 1.2em;
}
}
```

Start by reproducing the responsive line-height example with the default parameters and trace how those defaults are read when generating the calc() expression. Done means the example produces valid CSS without undefined, NaN, or the unsupported-units error while retaining the expected media-query values.

Written by the indexing model from the issue text.