Lua AI utility functions: fix for chance-to-hit specials without id

All mainline chance-to-hit weapon specials have ids, but it is not required for UMC specials.

data/ai/lua/battle_calcs.lua

@@ -54,7 +54,7 @@ function battle_calcs.unit_attack_info(unit, cache)
             else
                 -- magical, marksman
                 if (sp[1] == 'chance_to_hit') then
-                    a[sp[2].id] = true
+                    a[sp[2].id or 'no_id'] = true
                 else
                     a[sp[1]] = true
                 end