MT#63605 fix GET invoicetemplates without reseller_id

* 'reseller' level invoice templates are correctly
  returned with reseller_id: null, instead of 500 error

Change-Id: Ied1e7626fcbb2c28bf3850dd1788f06574ef892b
(cherry picked from commit 0d3aac99a7)
(cherry picked from commit ed71b62616)

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

@@ -46,7 +46,9 @@ sub hal_from_item {
             Data::HAL::Link->new(relation => 'collection', href => sprintf("/api/%s/", $self->resource_name)),
             Data::HAL::Link->new(relation => 'profile', href => 'http://purl.org/sipwise/ngcp-api/'),
             Data::HAL::Link->new(relation => 'self', href => sprintf("%s%d", $self->dispatch_path, $item->id)),
-            Data::HAL::Link->new(relation => 'ngcp:resellers', href => sprintf("/api/resellers/%d", $item->reseller_id)),
+            $item->reseller_id
+                ? Data::HAL::Link->new(relation => 'ngcp:resellers', href => sprintf("/api/resellers/%d", $item->reseller_id))
+                : (),
         ],
         relation => 'ngcp:'.$self->resource_name,
     );