Lua AIs: do not use empty tables as filters

It’s faster not to provide any argument to wesnoth.get_units() if all
units are to be found.

data/ai/lua/ai_helper.lua

@@ -1195,7 +1195,7 @@ function ai_helper.get_attacks(units, cfg)
 
     -- Note: the remainder is optimized for speed, so we only get_units once,
     -- do not use WML filters, etc.
-    local all_units = wesnoth.get_units {}
+    local all_units = wesnoth.get_units()
 
     local enemy_map, my_unit_map, other_unit_map = LS.create(), LS.create(), LS.create()
     for i,unit in ipairs(all_units) do

data/ai/micro_ais/cas/ca_bottleneck_move.lua

@@ -185,7 +185,7 @@ local function bottleneck_move_out_of_way(unit_in_way, self)
 
    local reach = wesnoth.find_reach(unit_in_way)
 
-   local all_units = wesnoth.get_units { }
+   local all_units = wesnoth.get_units()
    local occ_hexes = LS:create()
    for _,unit in ipairs(all_units) do
        occ_hexes:insert(unit.x, unit.y)