If a sniper w/ Pierce rolls a hit and there are no keywords on the defenders card that block Pierce, is that an automatic hit no matter what?
For example a Rebel sniper rolls a hit on a stormtrooper unit. Stormtroopers don’t have anything to counter Pierce. Since you would only roll 1 defense die, which would be canceled by Pierce, the shot would go through whether you rolled the die or not. Is this correct?

It's correct. Technically the defender has to roll the die, but if it's a block it will be cancelled by Pierce, so in that particular case it's the same as not rolling defense at all.

Pierce will cancel the first block results rolled. If no dice are rolled (if the hits are removed by cover for, example) then Pierce has no effect.