Fix data connectors options with object-like default values

[lorenzo-cavazzi]

There are a few options that have an object as default (E.G. try picking S3 / AWS and check the list_url_encode object); the storage schema contains something like "default": { "value": false, "valid": false } and that shows up as "[object Object]". The easiest way to handle that is removing the default (it's usually only for properties with very niche use case and we can address that better should users raise the problem).