Lua AIs: no need to set variables to nil explicitely

This is mostly done for consistency across the Lua AI code

(cherry-picked from commit 0e7c09f7b5)

data/ai/lua/generic_recruit_engine.lua

@@ -111,7 +111,7 @@ return {
                 -- In several cases this function only approximates the correct value (eg Thunderguard vs Goblin Spearman has damage capped by target health)
                 -- In some cases (like poison), this approximation is preferred to the actual value.
                 local best_damage = 0
-                local best_attack = nil
+                local best_attack
                 local best_poison_damage = 0
                 -- Steadfast is currently disabled because it biases the AI too much in favour of Guardsmen
                 -- Basically it sees the defender stats for damage and wrongfully concludes that the unit is amazing
@@ -548,7 +548,7 @@ return {
                 end
             end
             -- Correct count of units for each range
-            local most_common_range = nil
+            local most_common_range
             local most_common_range_count = 0
             for range, count in pairs(attack_range_count) do
                 attack_range_count[range] = count/enemy_type_count
@@ -562,7 +562,7 @@ return {
                 attack_type_count[attack_type] = count/enemy_type_count
             end
 
-            local recruit_type = nil
+            local recruit_type
             local leader = wesnoth.get_units { side = wesnoth.current.side, canrecruit = 'yes' }[1]
             repeat
                 recruit_data.recruit.best_hex, recruit_data.recruit.target_hex = ai_cas:find_best_recruit_hex(leader, recruit_data)
@@ -779,7 +779,7 @@ return {
             end
             local healer_count, healable_count = get_unit_counts_for_healing()
             local best_score = 0
-            local recruit_type = nil
+            local recruit_type
             local offense_weight = 2.5
             local defense_weight = 1/hp_ratio^0.5
             local move_weight = math.max((distance_to_enemy/20)^2, 0.25)

data/ai/lua/generic_rush_engine.lua

@@ -574,7 +574,7 @@ return {
             })[1]
 
             local villages = wesnoth.get_villages()
-            local target, best_cost = nil, AH.no_path
+            local best_cost, target = AH.no_path
             for i,v in ipairs(villages) do
                 if not wesnoth.match_location(v[1], v[2], { {"filter_owner", { {"ally_of", { side = wesnoth.current.side }} }} }) then
                     local path, cost = wesnoth.find_path(unit, v[1], v[2], { ignore_units = true, max_cost = best_cost })

data/ai/lua/retreat.lua

@@ -114,7 +114,7 @@ function retreat_functions.get_retreat_injured_units(healees, regenerates)
 
     local healing_locs = retreat_functions.get_healing_locations()
 
-    local max_rating, best_loc, best_unit = - math.huge, nil, nil
+    local max_rating, best_loc, best_unit = - math.huge
     for i,u in ipairs(healees) do
         local possible_locations = wesnoth.find_reach(u)
         -- TODO: avoid ally's villages (may be preferable to lower rating so they will

data/campaigns/Eastern_Invasion/ai/ca_ogres_flee.lua

@@ -29,7 +29,7 @@ function ca_ogres_flee:execution()
     local enemies = wesnoth.get_units {  { "filter_side", { {"enemy_of", {side = wesnoth.current.side} } } } }
     local enemy_attack_map = BC.get_attack_map(enemies)
 
-    local best_hex, best_unit, max_rating = {}, nil, - math.huge
+    local max_rating, best_hex, best_unit = - math.huge, {}
     for i,u in ipairs(units) do
         local reach = wesnoth.find_reach(u)