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)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D21409