network: allow an empty gateway address in the connection editor

It appears that this has always been the intention of the code,
but the check was incomplete.

https://bugzilla.gnome.org/show_bug.cgi?id=724312

panels/network/connection-editor/ce-page-ip4.c

@@ -756,7 +756,7 @@ ui_to_setting (CEPageIP4 *page)
                         goto out;
                 }
 
-                if (text_gateway && inet_pton (AF_INET, text_gateway, &tmp_gateway) <= 0) {
+                if (text_gateway && *text_gateway && inet_pton (AF_INET, text_gateway, &tmp_gateway) <= 0) {
                         goto out;
                 }