Update `dab.draft.schema.json` with null support for more properties to pass `dab validate`

[abhishekkumams]

Creating an issue to track schema validation for nullable fields in the config file. I imagine this is an issue for any field we consider nullable. So more than just ttl-seconds

Originally posted by @seantleonard in #2267 (comment)

Need to update the dab.draft.schema.json and add test validating schema satisfying the null support for those columns