For `[^\x00-\xff]`, while it is still treated as a full Unicode
character class, it is not empty. For instance `≥` would still be
matched.
However, when `CharClass::to_byte_class` is called on it (as is done
when using `regex::bytes::Regex::new` rather than `regex::Regex::new`),
it _is_ now empty, since it excludes all possible bytes.
This commit adds a test asserting that `regex::bytes::Regex::new`
returns `Err` for this case (in accordance with
https://github.com/rust-lang-nursery/regex/issues/106) and adds an
`is_empty` check to the result of calling `CharClass::to_byte_class`,
which allows the test to pass.
b96e5cb899