undefined might come as a string (#1196)
undefined might come as a string (#1196)

so a hack to check and cast it to a real undefined

Co-authored-by: Álvaro Manera <6eb5d7a89c921c53a7c5b092d0ed4760ec184dc8@cartodb.com>
CartoDB/Windshaft-cartodb,CartoDB/Windshaft-cartodb,CartoDB/Windshaft-cartodb,CartoDB/Windshaft-cartodb