Distinguish between no default value specified and a default value null specified in the OpenAPI.

[simon-spinner]

Schema::getDefault() today returns a null value if no default value is specified in the OpenAPI document. The OpenAPI standard allows to explicitly specify null when a property is nullable. However this case cannot be detected because Schema:getDefault() will return null in that situation, too. An additional boolean flag Schema::hasDefaultValue() would be required.

Use case is the OpenAPI generator, where I would like to generate special code if the default value is set explicitly to null.