Refine default value for `nullable` property in Schema

[cebe]

Looks like the default value of nullable should depend on whether type is set.

A true value adds "null" to the allowed type specified by the type keyword, only if type is explicitly defined within the same Schema Object.

See also discussion at:

• Validation issue when using allOf with $ref thephpleague/openapi-psr7-validator#118
• nullable oneOf/anyOf value rejected thephpleague/openapi-psr7-validator#146