[5.7] Fix wrong class being used when eager loading nullable MorphTo with withDefault()

[bucaneer]

Fixes #27369

Currently, MorphTo inherits the newRelatedInstanceFor() method from BelongsTo, which ignores the passed $parent argument and instantiates a related model based on the generalized relationship instance (without constraints) produced by the query builder . However, in the case of MorphTo relations, the no-constraint relationship instance does not know anything about related classes, and wrongly defaults to the parent model class. This commit simply makes use of the $parent argument to produce a related instance that is appropriate for the parent model instance.

[taylorotwell]

I am closing this pull request because it lacks sufficient tests. It is difficult for us to merge pull requests without these things because the change may introduce breaking changes to the framework.

Feel free to re-submit your change with a thorough explanation of the feature and tests - integration tests are preferred over unit tests.

Thanks!