Widgets: Correct the logic for displaying the `after_widget` parameter of the Archives widget.

Follow-up to [48349].

Props mukesh27, joyously.
Fixes #50609.
Built from https://develop.svn.wordpress.org/trunk@48409


git-svn-id: https://core.svn.wordpress.org/trunk@48178 1a063a9b-81f0-0310-95a4-ce76da25c4cd