test_rdoc_rdoc.rb: use File::NULL
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38341 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
test_rdoc_rdoc.rb: use File::NULL
use File::NULL for portability if possible.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38341 b2dd03c8-39d4-4d8f-98ff-823fe69b080e