feat: toggle strict null parent root nodes

[gp-lnuff]

As per #212, here is a draft of my suggestion.
By using the strictNullParentRootNodes() configuration method, the buildTree() method will not turn orphaned child nodes into root nodes. An hasStrictNullParentRootNodes() helper has been added to this end.
Needs documentation update.
Consider calling the configuration method by default in relationship() on a major version change.

[CodeWithDennis]

Thanks for the work on this! I think its a good solution for this version and like you said maybe in a major version we can set it as default?

[gp-lnuff]

Indeed, it would be as simple as adding the helper method call to the relationship() method - if we only have to set $strictNullParentRootNodes to true, we can just do that directly, but with the helper method we could even use a Closure to have more fine-grained control of the behaviour depending on the relationship() method call, such as relationship type, or whether the queries have been modified.