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

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

Start with dab.draft.schema.json and review the config properties that should accept null, using ttl-seconds as the existing example. Add the requested schema-validation test and run dab validate; done means nullable fields pass validation without weakening validation for non-null values.

Written by the indexing model from the issue text.