Always return zero in dataset count query if no results

Author: espona

federated-catalog/src/main/java/org/eclipse/edc/heleade/federated/catalog/extension/store/mongodb/cache/MongodbFederatedCatalogCache.java

@@ -249,7 +249,8 @@ private String countInternalDatasets(MongoClient connection, QuerySpec querySpec
         }
 
         var document = documents.first();
-        String result = document == null ? "{}" : document.toJson();
+        String result = document == null ? "{\"count\": 0}" : document.toJson();
+
         return result;
     }