Useless continue link for upgrade page

[filosofo]

If you request the upgrade page directly, e.g. site.com/wp-admin/upgrade.php, then the "backto" argument is empty. With current behavior, when backto is empty, WP sets the "continue" link to an empty string, which leads you to the same page you're already on.

So the workflow ends up being:

• request upgrade page
• click "upgrade wp database" link
• see "upgrade complete"; click "continue" link
• see "no upgrade required"; click continue link
• end on home page

The fourth step is unnecessary and removed by the patch, which sets continue link to home when backto is empty.

[dd32]

See: [12744] Always set a goback link on upgrade page. Props filosofo. Fixes #10986