FIX: origins_to_regexes should always return an array (PR #15589)

If the SiteSetting allowed_onebox_iframes contains a value of *, it will use the values of all_iframe_origins during the Oneboxing process. If all_iframe_origins itself contains a value of *, origins_to_regexes will try to return a “catch-all” regex.

Other code assumes origins_to_regexeswill return an array, so this change ensures the * case will return an array containing only the catch-all regex.