Formatting: Make the check for empty text in `wp_trim_excerpt()` more resilient.

This addresses a regression in [47808], which caused excerpts to be generated from post content if an empty string is passed, but not for other values considered empty, e.g. `null` or `false`.

Props riaanlom, laxman-prajapati, SergeyBiryukov.
Merges [48817] to the 5.5 branch.
Fixes #51042.
Built from https://develop.svn.wordpress.org/branches/5.5@48819


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48581 1a063a9b-81f0-0310-95a4-ce76da25c4cd