If the attack fails to do damage, then the death attack fails. (On preview, I see Legildur has quoted the appropriate rule already.)
If the attack does beat the DR, by even one point, then the death attack goes off exactly as normal. The save DC is not changed, because it depends only on the assassin's level and Int modifier.
Maybe you're thinking of coup de grace? The save DC there is based on damage dealt, so any DR will effectively help you save against a CDG. This has nothing to do with death attacks as such, though.