test_m17n.rb: nil replacement

* test/ruby/test_m17n.rb (test_scrub, test_scrub_bang): now explicit
  nil is allowed as replacement.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43501 b2dd03c8-39d4-4d8f-98ff-823fe69b080e