Use `null` for `Import::$mediaQuery` to indicate absence

[JakeQZ]

The code currently uses both null and an empty string to indicate there is no media query. It also appears that when it is an empty string, an extra space will be rendered.

We should consistently use one or other, and it's probably cleaner to use null to indicate absence.

[JakeQZ]

We could also make the $mediaQuery parameter of the constructor optional.