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 https://github.com/Azure/data-api-builder/pull/2267#discussion_r1646317383

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