PHP 7.3 Compatibility: Fix compact related notices.

In PHP 7.3, the `compact()` function has been changed to issue an `E_NOTICE` level error if a passed string refers to an unset variable. In previous versions of PHP, this notice was silently skipped. This fixes a few more instances of unset variables in the WordPress admin.

The full RFC can be viewed here: https://wiki.php.net/rfc/compact.

See #44416.

Merges [44185] into trunk.

Fixes #45483.
Built from https://develop.svn.wordpress.org/trunk@44297


git-svn-id: https://core.svn.wordpress.org/trunk@44127 1a063a9b-81f0-0310-95a4-ce76da25c4cd