ignore cache keys that are not page ids

[bnomei]

if the page object here is null then return early. this allows other code to hijack the pages cache and add key-value that do not relate to a real page object. like sitemaps.
why hijack? so these cache values get flushed when the pages cache does.

staticache/src/StatiCache.php

Line 107 in 9ef2d71

$url = $page->url($key['language']);

[bnomei]

oh my. duplicate of this. #20 (comment)