Coding Standards: Correct the `get_plugin_page_hook()` check in `wp-admin/admin.php`.

The condition is meant to check for a non-empty string, however `get_plugin_page_hook()` can return null, in which case the strict check doesn't work as expected.

Follow-up to [47218].

See #48455, #49222.
Built from https://develop.svn.wordpress.org/trunk@47220


git-svn-id: https://core.svn.wordpress.org/trunk@47020 1a063a9b-81f0-0310-95a4-ce76da25c4cd