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.

[rubenvanassche]

Can you add a tests which checks that indeed the optional value is replaced with null and also checks what happens when the option is on and we've got a type like string|Optional which doesn't support Optional?

[ragulka]

Can you add a tests which checks that indeed the optional value is replaced with null and also checks what happens when the option is on and we've got a type like string|Optional which doesn't support Optional?

@rubenvanassche sure thing - added in b22cc7b.

Note that when using string|Optional, the property will end up not being set. When converting such DTO to array, it will simply exclude that property from the array - which I believe is to be expected.

However, the property must not be accessed before initialization, or PHP will throw an error:

Error: Typed property SongData::$slug must not be accessed before initialization

This might throw off some, so I've added a warning to docs about this as well..