This all happened Long, Long Ago -- much closer to the Big Bang. There is still much denser cosmic microwave background radiation for the TIE panels to pick up and use for energy. Also, there are polyenergized ions in the interstellar medium (the same medium that transmits sounds!) which can be converted into propulsion by the TIE engines via the Empire's ubiquitous Ion schmoozing process. This insterstellar medium (known to some as phlogistan) transmits Force waves as well.
Source? me!
