As Pittsburgh braces for the impact of the touring Radio City Christmas Spectacular to hit local arts groups, arts leaders are discovering that there’s just no way to tell in advance what that impact will be. A holiday entertainment glut is a possibility, which might see ticketbuyers shunning the same old Nutcracker performances in favor of the shiny new Rockettes. But the expected 30% spike in winter heating costs could also cause sales slumps all around. In any case, the beleagured Pittsburgh Ballet Theatre doesn’t need the new headache.