When initially defaulting the screen `$id` in `WP_Screen::get()`, if the `$hook_name` parameter is not supplied, an `else` fallback uses `$GLOBALS['hook_suffix']`. However, in some cases, `hook_suffix` doesn't exist in the global scope. This produces an "Undefined index" notice on < PHP 8, and a warning in >= PHP 8.
This change ensures `$GLOBALS['hook_suffix']` has a value before using it as a fallback for the screen ID.
Props splendorstudio, SergeyBiryukov, htdat, mukesh27, dd32, costdev.
Fixes#49089.
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