i18n: Prevent a PHP warning when a mu-plugin loads a textdomain.

`determine_locale()` accesses the `$pagenow` global, but this is set after mu-plugins are loaded, so we need to check that it's been set.

Props swissspidy, azaozz.
Merges [44284] to trunk.
Fixes #45668.
Built from https://develop.svn.wordpress.org/trunk@44317


git-svn-id: https://core.svn.wordpress.org/trunk@44147 1a063a9b-81f0-0310-95a4-ce76da25c4cd