update_site_option can fail for values that cast to false

[duck_]

Passing a value to update_site_option that casts to false (e.g. 0) doesn't work when the site option doesn't already exist because of:

if ( $value && !$wpdb->get_row( $wpdb->prepare( "SELECT meta_value FROM $wpdb->sitemeta WHERE meta_key = %s AND site_id = %d", $option, $wpdb->siteid ) ) )
    return add_site_option( $option, $value );

update_option has no such restriction (the first part of the conditional).

Found when investigating #15496 as the compression test might call update_site_option('can_compress_scripts', 0);, so in some circumstances the compression test is loaded on every admin screen.

[duck_]

In [18589]:

Allow update_site_option to add false-y values. Fixes #15497