Lua AIs: fix bug occurring when number= in an attack is set to zero

Apparently, when this is the case, the number key is not included at
all, rather than set to zero, when the Lua table containing the attack
information is retrieved.

data/ai/lua/battle_calcs.lua

@@ -67,6 +67,9 @@ function battle_calcs.unit_attack_info(unit, cache)
             end
         end
 
+        -- 'number' key is apparently not included in attack table if its value is zero
+        a.number = a.number or 0
+
         table.insert(unit_info.attacks, a)
     end