Fix radiation protection when armor group is not set

The radiation damage group can be not set when armor completely
protects against radiation or when no mod has set a default value
for the damage group. We want to apply radiation damage in the
second case but not in the first.

technic/radiation.lua

@@ -294,6 +294,8 @@ local function calculate_damage_multiplier(object)
 	end
 	if ag.radiation then
 		return 0.01 * ag.radiation
+	elseif armor_enabled then
+		return 0
 	end
 	if ag.fleshy then
 		return math.sqrt(0.01 * ag.fleshy)