"Zoolander 2" employed the talents of four credited screenwriters, but unfortunately not one of them seemed to notice that what they were working on wasn't actually a screenplay. It is, instead, an excuse to string together an endless and often random supply of celebrity cameos (Willie Nelson, why are you in this movie?) and to reunite a pair of beloved but played-out characters from the original 2001 "Zoolander." The result: a mostly agreeable but empty-headed mess. It's sort of the movie equivalent of Derek Zoolander himself.