fix spurious config write, closes https://github.com/opnsense/core/issues/1029

(cherry picked from commit 0e38a487)

src/etc/inc/plugins.inc

@@ -122,7 +122,7 @@ function plugins_interfaces()
                     $intf_config['internal_dynamic'] = true;
                     // traverse and diff interface properties with known configuration
                     foreach ($intf_data as $prop_name => $prop_value) {
-                        if (empty($intf_config[$prop_name]) || $intf_config[$prop_name] != $prop_value) {
+                        if ((empty($intf_config[$prop_name]) && !empty($prop_value)) || $intf_config[$prop_name] != $prop_value) {
                             $intf_config[$prop_name] = $prop_value;
                             if (!in_array($intf_ref, $changed_interfaces)) {
                                 $changed_interfaces[] = $intf_ref;