That's not true. There isn't anything special about jury nullification. If it happens, it happens and that's the end of the trial. If the jury is hung because some of the jury members wish to nullify and others don't, then it will lead to a mistrial simply because the jury cannot come to a unanimous decision - not because of jury nullification. Of course, any verdict can be appealed as usual, but there's no guarantee the appeal will be granted - even in the case of jury nullification.