calling Tilt[] with unregistered file type shouldn't add empty mapping

If you call `Tilt[]` with an unregistered file type, it correctly returns `nil`, but also unexpectedly adds a mapping for `"" => []`. This happens because an empty string is passed to `Tilt.template_mappings[]`, and it creates a default value for that key of `[]`.

This pull request fixes that bug and includes a test case.