Previously, when the `do-plugin-upgrade` or `do-theme-upgrade` actions were accessed directly on `update-core.php`, an error message stating "Select one or
more (plugins|themes) to update" would be shown even if there was nothing to update.
This ensures that the error message only appears when there is something to update.
Props Presskopp, sabernhardt, pbiron, afragen, zunaid321, shuvoaftab, ugyensupport, costdev.
Fixes#57999.
Built from https://develop.svn.wordpress.org/trunk@56107
git-svn-id: http://core.svn.wordpress.org/trunk@55619 1a063a9b-81f0-0310-95a4-ce76da25c4cd