Commit message:
> ```
> MFC 351481, 357619:
>
> Approved by: bcr (mentor)
> Differential Revision: https://reviews.freebsd.org/D24058
>
> mixer(8): Report an error if the passed value is an empty string
>
> This patch fixes a bug that made the mixer command enter
> an infinite loop when instructed to set the value of a device
> to an empty string (e.g., `mixer vol ""`).
>
> Additionally, some tests for mixer(8) are being added.
>
> PR: 240039
> Reviewed by: hselasky, mav
> Approved by: src (hselasky, mav)
> Differential Revision: https://reviews.freebsd.org/D21409
>
>
> mixer: call the cleanup function in a test
>
> The set_empty_value test has a cleanup function, but is not called.
> Fix it
>
> Reviewed by: 0mp
> Approved by: kp
> Differential Revision: https://reviews.freebsd.org/D23498
> ```