Upgrade/Install: Make the check for empty `home` option in `__get_option()` more resilient.

This addresses a regression in [47808], where the `home` check expected an empty string to use `siteurl` as a fallback, but `wpdb::get_var()` returns `null` if the option is empty.

Props fjarrett.
Merges [48868] to the 5.5 branch.
Fixes #51011.
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git-svn-id: https://core.svn.wordpress.org/branches/5.5@48631 1a063a9b-81f0-0310-95a4-ce76da25c4cd