Administration: Don't print the `.php-error` class in the admin header for PHP...

Administration: Don't print the `.php-error` class in the admin header for PHP notices in `wp-config.php`.

Any notices in that file happen before `WP_DEBUG` takes effect, and should not be displayed with the `error_reporting` level previously set in `wp-load.php`.

Adding the class when there is no notice displayed results in extra padding under the admin bar, which should be avoided.

Follow-up to [20102], [21025], [26620], [47745].

Props akissz, tw2113, laxman-prajapati, sabernhardt, johnbillion, SergeyBiryukov.
Merges [48850] to the 5.5 branch.
Fixes #51073.
Built from https://develop.svn.wordpress.org/branches/5.5@48851


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48613 1a063a9b-81f0-0310-95a4-ce76da25c4cd