Fix re-triggering scheduled products with falsy required params

It is possible to set required parameters to falsy values when
scheduling a product, e.g. `VERSION=0`. This changes allows
re-triggering such a scheduled product without running into a
validation error.

lib/OpenQA/WebAPI/Controller/API/V1/Iso.pm

@@ -129,7 +129,7 @@ sub create {
         }
         my $settings_to_clone = $previously_scheduled_product->settings // {};
         for my $required_param (MANDATORY_PARAMETERS) {
-            if (!$settings_to_clone->{$required_param}) {
+            if (!defined $settings_to_clone->{$required_param}) {
                 return $self->render(
                     text => "Scheduled product to clone settings from misses $required_param.",
                     status => 404