Upgrade/Install: Check if the theme installer skin's `overwrite` property...

Upgrade/Install: Check if the theme installer skin's `overwrite` property exists in `Theme_Upgrader::install_strings()`.

This ensures consistency with `Plugin_Upgrader::install_strings()` and resolves an issue caused by the property not existing in other upgrader implementations.

Props schlessera, azaozz.
See #50670.
Built from https://develop.svn.wordpress.org/trunk@48493


git-svn-id: https://core.svn.wordpress.org/trunk@48255 1a063a9b-81f0-0310-95a4-ce76da25c4cd