def serialize_unknown_attributes?
      @serialize_unknown_attributes.nil? ? true : @serialize_unknown_attributes
    end
Returns the value of the `@serialize_unknown_attributes` instance
variable. In the current specification, unknown attributes must be
persisted in the database regardless of the globally configured
`serialize_unknown_attributes` option. Therefore, it returns the
default value `true` if the instance variable is `nil`.

This method is used to ensure that the `serialize_unknown_attributes`
option is correctly applied to nested StoreModel::Model objects when
the `as_json` method is called.

@return [Boolean]