[ruby/irb] Make sure ls doesn't return an array

(https://github.com/ruby/irb/pull/461)

https://github.com/ruby/irb/commit/367797bb05

lib/irb/cmd/ls.rb

@@ -30,6 +30,7 @@ module IRB
         o.dump("instance variables", obj.instance_variables)
         o.dump("class variables", klass.class_variables)
         o.dump("locals", locals)
+        nil
       end
 
       def dump_methods(o, klass, obj)