Widgets: Always try restoring widget assignments

For code-historic reasons, restoring widget assignments would not be attempted if there were no sidebars to map.
Restoring previous assignments is something unrelated to sidebar mappings however, so now it will be attempted on every theme switch.

See #42719.

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git-svn-id: https://core.svn.wordpress.org/trunk@42203 1a063a9b-81f0-0310-95a4-ce76da25c4cd