systemd.bbclass: allow recipe to inherit systemd with empty SYSTEMD_SERVICE

* I have use case where common recipe does inherit systemd and only some BSP's are bbappending some SERVICEs to it
* install was moved intentionaly, so that /lib/systemd/system is not
  created if there is no .service file (causing QA Warning about
  unpackaged dir)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>