Does a defender need to be present to have an attacker declared?

Yes, a defender must be present to have an attacker declared.

According to Rule 459.2.b (Steps of Combat) and RiftJudge FAQ #7651 / #10095, the designations "Attacker" and "Defender" are only established when a Combat occurs. Combat specifically requires units from two opposing players to be present at the same battlefield.

If you move a unit to an unoccupied battlefield, it initiates a "non-combat" Showdown. Because there is no opposing unit present to create a combat encounter, no one receives the "Attacker" or "Defender" designation. Consequently, keywords and abilities that rely on these designations—such as Assault (for attackers) or Shield (for defenders)—do not activate or apply in that scenario.