Add a test for empty strings

Empty strings are currently parsed as arrays. I only wrote the test because I'm unsure of how to fix it.

It appears that the empty string is being parsed as a key. The following rule gets a subtree `v` that is `{string: []}`: https://github.com/jm/toml/blob/fced48ec1b30daa3b89032f005d8e125d5c75950/lib/toml/transformer.rb#L79-L81