small illuminates optimisation

removes a call to get_ability_bool, in case that get_ability_bool
would return false, it would ot be faster than get_abilities, in
case that it returns true it is useless.

src/tod_manager.cpp

@@ -231,22 +231,22 @@ const time_of_day tod_manager::get_illuminated_time_of_day(const unit_map & unit
 		get_adjacent_tiles(loc, locs.data() + 1); // start at [1]
 		for ( std::size_t i = 0; i < locs.size(); ++i ) {
 			const unit_map::const_iterator itor = units.find(locs[i]);
-			if (itor != units.end() &&
-			    itor->get_ability_bool("illuminates") &&
-			    !itor->incapacitated())
+			if (itor != units.end() && !itor->incapacitated())
 			{
 				unit_ability_list illum = itor->get_abilities("illuminates");
-				unit_abilities::effect illum_effect(illum, terrain_light, false);
-				const int unit_mod = illum_effect.get_composite_value();
-
-				// Record this value.
-				mod_list.push_back(unit_mod);
-				max_list.push_back(illum.highest("max_value").first);
-				min_list.push_back(illum.lowest("min_value").first);
-				if ( unit_mod > most_add )
-					most_add = unit_mod;
-				else if ( unit_mod < most_sub )
-					most_sub = unit_mod;
+				if(!illum.empty()) {
+					unit_abilities::effect illum_effect(illum, terrain_light, false);
+					const int unit_mod = illum_effect.get_composite_value();
+
+					// Record this value.
+					mod_list.push_back(unit_mod);
+					max_list.push_back(illum.highest("max_value").first);
+					min_list.push_back(illum.lowest("min_value").first);
+					if ( unit_mod > most_add )
+						most_add = unit_mod;
+					else if ( unit_mod < most_sub )
+						most_sub = unit_mod;
+				}
 			}
 		}
 		const bool net_darker = most_add < -most_sub;