TT#22505 safe handling of "" lock level pref values

Change-Id: I46b0e507daabe1c3af2412cdae1bfe403a85286b
(cherry picked from commit 443426674f)

lib/NGCP/Panel/Role/API/Subscribers.pm

@@ -127,9 +127,9 @@ sub resource_from_item {
             c => $c, attribute => 'lock',
             prov_subscriber => $item->provisioning_voip_subscriber);
         $resource{lock} = 0;
-        if($pref->first) {
+        if($pref->first and length($pref->first->value) > 0) {
             #cast to Numeric accordingly to the form field type and customer note in the ticket #10313
-            $resource{lock} += $pref->first->value;
+            $resource{lock} = $pref->first->value;
         }
     } else {
         # fields we never want to see