When serializing an OpenApiSchema object as 3.1+, the Const property is ignored completely if it's set to null. However, 'null' value is allowed by the draft 2020-12 spec.

There should be two concepts, first is "unset value" and second is "value set to null".

In addition to that, when serializing Const as 2.0 or 3.0, the same bug exist, but also an empty string value is considered as "unset". Empty string is always clearly a value that should be serialized and should never be treated as "unset".

I imagine this might need a new public API like OpenApiUnsetValues.UnsetString so users can do:

var schema = new OpenApiSchema();
schema.Const = null; // serializes to null.
schema.Const = OpenApiUnsetValues.UnsetString; // doesn't serialize.

The default value of Const will be OpenApiUnsetValues.UnsetString.

I'm not sure if there are any other properties having similar issues as well.

@baywet any thoughts?