getremoteconfaddr cannot be set to HTTP_CLIENT_IP, HTTP_X_FORWARDED_FOR, REMOTE_ADDR

A change in defaults set with MDL-69074 means it is now impossible to set CFG->getremoteaddrconf to HTTP_CLIENT_IP, HTTP_X_FORWARDED_FOR, REMOTE_ADDR (value: 0).

MDL-69074 did this:

    if (empty($CFG->getremoteaddrconf)) {

         // This will happen, for example, before just after the upgrade, as the

         // user is redirected to the admin screen.

-        $variablestoskip = 0;

+        $variablestoskip = GETREMOTEADDR_SKIP_DEFAULT;

     } else {

         $variablestoskip = $CFG->getremoteaddrconf;

     }

The problem is "0", which is used by "HTTP_CLIENT_IP, HTTP_X_FORWARDED_FOR, REMOTE_ADDR" equates to empty - and the empty branch of the IF no longer sets the value to 0.

We should change `if (empty(...` to `if (!isset(...` - which allows the UI/config.php to set the value to '0'.