WordArray allows all characters except spaces

Fixes #1352.

WordArray would not register an offense for `['a@example.com', 'b@example.com']`, because it checked that each string should consist of word characters. So an email address containing `@` and `.` would not be a candidate for a `%w()`-style array.

As far as I can see, it should be enough to just check that the strings don’t contain spaces.
Why is the `.empty?` check needed?