Fixed thrown exception in case the type does not exists to return a 404

Not Found HTTP error to client

src/main/java/org/gcube/informationsystem/resourceregistry/types/SchemaManagementImpl.java

@@ -383,6 +383,9 @@ public class SchemaManagementImpl implements SchemaManagement {
 			OMetadata oMetadata = oDatabaseDocument.getMetadata();
 			OSchema oSchema = oMetadata.getSchema();
 			OClass baseOClass = oSchema.getClass(typeName);
+			if(baseOClass == null) {
+				throw new SchemaNotFoundException(typeName + " does not Exists");
+			}
 			
 			List<Type> typeDefinitions = new ArrayList<>();
 			typeDefinitions.add(getType(baseOClass));