Require value for @Parent when eager loaded

[tanner0101]

Queries that fail to eager-load a value for a @Parent relation will now throw an error (#380).

This change helps prevent unintended access of uninitialized @Parent properties in cases where no parent is found. For example, if the parent is deleted (including soft-deletion) or the reference was set to an invalid identifier.

[tanner0101]

One potential caveat with this change is that users could have been relying on this type of query not resulting in an error. It is possible to check if the parent was eager loaded using model.$parent.value instead of accessing directly. However, a state where the value in an @Parent field does not point to an existing model is technically invalid. Especially given this can cause uninitialized access assertions, it seems that Fluent should prevent this where possible. As a future direction, it may make sense to allow for eager loading to fail silently via a flag to recreate the old behavior. This would allow (careful) use of QueryBuilder even if the data is in an invalid state.

[tanner0101]

These changes are now available in 1.7.3