Widgets: Don't try mapping empty sidebars.

Fixes a bug where the mapping logic would try mapping empty sidebars, resulting in PHP warnings.

Props ionvv, chetan200891 for initial patch.
Fixes #42603.


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git-svn-id: https://core.svn.wordpress.org/trunk@42191 1a063a9b-81f0-0310-95a4-ce76da25c4cd