Resolve question about presence/absence of OpenAPI object properties

[timburks]

Question about object properties from @wora:

Does OpenAPI depend on presence? e.g. for a property like "bool foo", does OpenAPI treat true, false, and null as 3 separate states? If you use proto3, you will prohibit swagger from allowing such tri-state properties. To support tri-state properties, you have to use proto2.

[timburks]

All boolean values in the 2.0 spec and 3.0 draft default to false.