There’s nothing worse than reviewing a mediocre film made up of a trio of elements I actually like. In that respect, Black Nativity is a compressed version of my personal hell; it’s helmed by Kasi Lemmons, the gifted director behind such treasures as Talk to Me and Eve’s Bayou, it boasts a phenomenal cast that begins with Forest Whitaker (nearly ubiquitous in 2013) and ends with Angela Bassett, and it calls on the works of the great Langston Hughes to serve as its foundation. But none of that winds up mattering much, because ultimately Black Nativity winds up doing little more than just existing; it’s there, but it’s not especially good. If you’re a glass half-full type, that suggests the…