Fix Emacs 27.1 incompatibility in lyskom-subtract-one-day

Keep any sub-second time elements unchanged, so that we return a valid
time format in lyskom-subtract-one-day.  Emacs 27.1 expects it to be
on the (HI LO US PS) form, and decode-time will raise an error if it
is on the (HI LO nil) form that lyskom-subtract-one-day previously
returned.