ExpAI recruit rushers: do not parse side __cfg each eval

data/ai/lua/ca_recruit_rushers.lua

@@ -30,7 +30,11 @@ local function ca_castle_switch()
         end
     end
 
-    return nil
+    -- Avoid going through the __cfg above every time the evaluation function
+    -- is called if the castle switch CA does not exist
+    local dummy_castle_switch = {}
+    function dummy_castle_switch:evaluation() return 0 end
+    return dummy_castle_switch
 end