Simply Platform#null_device to just use File::NULL

It was introduced in Ruby 1.9.3 way older that our minimum required version
(https://svn.ruby-lang.org/repos/ruby/tags/v1_9_3_0/NEWS)

rb/lib/selenium/webdriver/common/platform.rb

@@ -102,11 +102,7 @@ def cygwin?
       end
 
       def null_device
-        @null_device ||= if defined?(File::NULL)
-                           File::NULL
-                         else
-                           Platform.windows? ? 'NUL' : '/dev/null'
-                         end
+        File::NULL
       end
 
       def wrap_in_quotes_if_necessary(str)