hail hydra bugfix (#1558)

Fixed empty unit_group creation, leading to lag on large maps

Author: actinoide

map_gen/shared/hail_hydra.lua

@@ -151,6 +151,9 @@ local on_died =
             if group and group.valid and #group.members > 0 then
                 group.set_command(command)
             end
+            if group and group.valid and #group.members == 0 then
+                group.destroy()
+            end
         end
     end
 )