PHP7.3 compatibility: Fix compact throwing 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. The full RFC can be viewed here: https://wiki.php.net/rfc/compact.

Props jorbin, desrosj.

Merges [43819] and [43832] to trunk.

Fixes #44416.
Built from https://develop.svn.wordpress.org/trunk@44166


git-svn-id: https://core.svn.wordpress.org/trunk@43996 1a063a9b-81f0-0310-95a4-ce76da25c4cd