...instead of returning the one from its associated style sheet.
This reverts 848a250b29 where I made
`CSSImportRule.media` nullable.
CSSImportRule may not have an associated style sheet, because of not
matching a supports condition, or just failing to load the URL.
Regardless of whether we do or not, the expected (non-spec) behaviour
is that we should return a MediaList always, which matches the media
queries specified on the `@import` rule.
See the linked spec issue for more details. The MediaList can be null
internally, and this was upsetting GCC as it meant our bindings code
was dereferencing a null pointer.
