test_features.rb: fix up r46123

* test/csv/test_features.rb: no longer `nil` is valid data source
  for `CSV.new`.  [GH-580]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46125 b2dd03c8-39d4-4d8f-98ff-823fe69b080e