Moved a check in the DNA machine to hopefully fix that pesky "but there's nothing connected!" runtime.

Thanks Carn and CIB.

The following runtime has occured 7 time(s).
runtime error: Cannot read null.occupant
proc name: Topic (/obj/machinery/computer/scan_consolenew/Topic)
  source file: dna.dm,891
  usr: Trey Campbell (/mob/living/carbon/human)
  src: DNA Scanner Access Console (/obj/machinery/computer/scan_consolenew)