Settings: Replace `count()` call with `empty()` in `get_settings_errors()` to...

Settings: Replace `count()` call with `empty()` in `get_settings_errors()` to prevent PHP 7.2 warnings when `$wp_settings_errors` is `null`.

Props pross, dd32, westonruter.
See #40109.
Fixes #42498 for trunk.

Built from https://develop.svn.wordpress.org/trunk@42146


git-svn-id: https://core.svn.wordpress.org/trunk@41977 1a063a9b-81f0-0310-95a4-ce76da25c4cd