Administration: Handle the result of `Plugin_Upgrader::bulk_upgrade()` for a...

Administration: Handle the result of `Plugin_Upgrader::bulk_upgrade()` for a plugin that is already at the latest version in the same way it is handled for themes.

This corrects a fragile check of the result in `wp_ajax_update_plugin()` that depended on the internal array pointer, and brings some consistency with `wp_ajax_update_theme()`.

Follow-up to [37714], [48401].
See #50448.
Built from https://develop.svn.wordpress.org/trunk@48445


git-svn-id: https://core.svn.wordpress.org/trunk@48214 1a063a9b-81f0-0310-95a4-ce76da25c4cd