Remove `null` for collections

[silasjoisten]

If a collection can be empty, then we don't need to allow null and don't need to use it as a default value in StoriesRequest, just use:

- public ?RelationCollection $withRelations = null,
+ public RelationCollection $withRelations = New RelationCollection(),

this would also simplify the check in StoriesRequest::toArray() then

Originally posted by @OskarStark in #8 (comment)