belongsTo/hasMany fetch by link with missing data key in returned payload should error.

[runspired]

When an async belongsTo or hasMany relationship with no known data (not known empty and not known not-empty but has a link) is fetched via link, an error should be thrown if the returned payload does not have a data key (no error should be thrown if the returned payload has a data key but it specifies null for belongsTo or specifies [] for hasMany).

See #7534 for a recent adjustment to this codepath that would be the likely point at which to assert this.

It's possible we do error today, but I didn't see a test at first glance.