If my opponent has no benched Pokemon, do I do 50 damage to their active Pokemon when I use Nidoking's "Drag Off" attack?
No. Damage is only done to the "new Active Pokemon". If there is no switch, then no damage is done.
Source: Team Up FAQ (2019-01-18), TPCi Rules Team (2019-01-31)

Relevant cards

Note: All applicable cards may not be listed below.
SM9_59
SM9_59