MT#55538 /api/soundsets allow filtering by customer_id=null

* when customer_id query param is specified as customer_id=null
  or customer_id=NULL it now correctly fetches soundsets that
  do not have customer_id assigned to them

Change-Id: I8de3d9615c133c2abd3eb2b5f4fea8de5b652417
(cherry picked from commit a86e868a6f)

lib/NGCP/Panel/Controller/API/SoundSets.pm

@@ -26,7 +26,7 @@ sub query_params {
             query => {
                 first => sub {
                     my $q = shift;
-                    return { 'contract_id' => $q };
+                    return { 'contract_id' => lc($q) eq 'null' ? undef : $q };
                 },
                 second => sub {},
             },