Drop update_region() call with NULL argument

That call does not fit into Ubuntu's model for handling the
relation between Language and Formats, and it causes warnings
like these:

(gnome-control-center:13073): GLib-CRITICAL **: 19:37:21.898: g_variant_new_string: assertion 'string != NULL' failed

(gnome-control-center:13073): region-cc-panel-WARNING **: 19:37:21.903: Couldn't set FormatsLocale: GDBus.Error:org.freedesktop.Accounts.Error.Failed: '[Invalid UTF-8]' is not a valid locale name

(gnome-control-center:13073): GLib-CRITICAL **: 19:37:21.916: g_regex_match_full: assertion 'string != NULL' failed

(gnome-control-center:13073): GnomeDesktop-WARNING **: 19:37:21.916: locale '(null)' isn't valid

So let's drop it as a follow-up of 4a8f9186.