tkutil.c: ary can be nil

• ext/tk/tkutil/tkutil.c (tk_hash_kv): the third argument can be
  nil not only an Array. reported by @windwiny at
  https://.com/ruby/ruby/commit/cdaa94e#commitcomment-17096618

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54575 b2dd03c8-39d4-4d8f-98ff-823fe69b080e