Allow disabling default Optional values in CreationContext

[ragulka]

DTOs that have properties using the Optional type can be a little painful to work with when creating the DTO in a context where you'd rather have a missing value be resolved to null. This can happen when preparing a DTO on the server-side, later to be converted to a JSON payload. For instance, when an Eloquent model is converted to array, it may not include all the keys for the DTO - which would result in Optional type being used for the matching properties.

This PR makes it possible to control whether the DefaultValuesPipeline will set properties that support the Optional type to Optional, or fall back to null (if allowed by the property).

See the docs diff for usage.