Fix traceback in service module when svc_cmd is None (2nd fix)

When service module is used on unsupported Linux system where init
script is used directly, LinuxService.svc_cmd is None so .endswith()
fails.

This extends fix from e2f20db53481128553d876109d5fbdab9f43dd5b also
for state=restarted.

Fixes issue #3533