This removes the argument if the specified toolchain is empty.
As far as I can tell there is no +nighly (or similar) option [1] leading to
the termination of the server. But since people needed this option and
have yet to complain about it it stays the default for now.
[1] https://github.com/rust-lang-nursery/rls/blob/master/src/main.rs#L87
w0rp