Warning about non-existing feature '' if --skip '' is specified

[Nemo157]

I'm setting up some reusable workflows for my projects and as part of that need to pass in a parameterized --skip flag. Previously on 0.3.9 having this empty for projects that don't need to skip any features worked without any warnings, now on 0.5.17 this outputs a warning:

> cargo hack check --feature-powerset --skip ''
warning: specified feature `` not found in package `self-test`
info: running `cargo check --no-default-features` on self-test (1/1)
    Finished dev [unoptimized + debuginfo] target(s) in 0.00s

It would be nice if an empty string was considered the same as not passing the flag.

[taiki-e]

Fixed in 0.5.18 (#164). Thanks for the report!