Fix error with empty args.

* ext/pathname/lib/pathname.rb (Pathname#join): Fix error with
  empty args. Reported by ko1 via IRC.

* test/pathname/test_pathname.rb (TestPathname#test_join): Add the
  test for above case.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45499 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-04-02

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45500 b2dd03c8-39d4-4d8f-98ff-823fe69b080e