openapi::schema::empty is confusing or wrong

[jayvdb]

https://docs.rs/utoipa/latest/utoipa/openapi/schema/fn.empty.html says

Create an empty Schema that serializes to null.
Can be used in places where an item can be serialized as null. This is used with unit type enum variants and tuple unit types.

It becomes a jsonschema

type: object
default: ~
nullable: true

Because it has type: object in there, it can not be used to describe an empty string, int, or any other type. It can only be used to describe an empty object.

It should be changed to use the new Value support added in #568