Avoid calling [] method multiple times

Author: brucehsu

lib/grape/util/stackable_values.rb

@@ -15,8 +15,8 @@ def [](name)
         return @frozen_values[name] if @frozen_values.key? name
 
         value = []
-        value.concat(@inherited_values[name]) if @inherited_values[name]
-        value.concat(@new_values[name]) if @new_values[name]
+        value.concat(@inherited_values[name] || [])
+        value.concat(@new_values[name] || [])
         value
       end