Does taking an empty zone at the beginning of the game allow enemy players to play actions and reactions to contest even though they don't have any units there?

Ruling: Yes, taking an empty zone still starts a showdown, and enemy players can play actions and reactions to contest even without units present.

Nuances:

• In multiplayer games (e.g., 4 players), every other player would be able to play spells to contest
• Playing a unit from hand onto a battlefield you control does not start a showdown
• You cannot play units from hand to a battlefield you don't control unless you have an ability that specifically allows it (like purple Miss Fortune)