Lua: Traps without victims

Traps may get corpses generated on them on early dungeon levels,
to warn off fragile starting heroes. Allow creating traps in lua
without the corpse.

doc/lua.adoc

@@ -929,6 +929,7 @@ Example:
  des.trap({ type = "hole", x = 1, y = 1 });
  des.trap({ type = "hole", coord = {2, 2} });
  des.trap({ type = "web", coord = {2, 2}, spider_on_web = false, seen = true });
+ des.trap({ type = "falling rock", victim = false });
  des.trap({ type = "rolling boulder", coord = {7, 5}, launchfrom = {-2, -2} });
  des.trap("hole", 3, 4);
  des.trap("level teleport", {5, 8});