More of a holiday dramedy than a jolly seasonal RomCom, director Clea DuVall’s loosely autobiographical “Happiest Season” seeks to invert the traditional Christmas movie tropes but inadvertently finds itself trapped by the conventions of the genre and a too glib approach to seasonal redemption. Abby (Kristen Stewart) and Harper (McKenzie Davis) have been dating for a year and are madly in love.…