Clarify how/if null property values are supported

[cipresso]

Hi. One of the things we're hoping for in the OpenAPI spec is support for null types. This is especially important for SQL where "property":"" means something different from "property":null. Using JSON schema you indicate something can be null by using a type array as in "type":['string','null']. What would be the equivalent in the next version of OpenAPI?

[darrelmiller]

The current plan is to have null support. See #741

[cipresso]

Thanks Darrel!