Summary:
#16008 happened because we forgot to typecheck nullary
associated type family instances in a way that's consistent with the
type variables bound by the parent class. Oops. Easily fixed with a
use of `checkConsistencyConstraints`.

Test Plan: make test TEST=T16008

Reviewers: simonpj, goldfire, bgamari

Reviewed By: goldfire

Subscribers: rwbarton, carter

GHC Trac Issues: #16008

Differential Revision: https://phabricator.haskell.org/D5435