When Master Negotiator enters the battlefield, you may gain control of target nonland permanent an opponent controls with mana value 2 or less. If you do, its controller may draw a card. Otherwise, you draw a card.
If there is no legal target for the etb ability, the ability won't go on the stack and you won't draw a card. Similarly if the target becomes illegal, the ability will be countered and you will not draw a card.