Is a unit considered attacking if there are no units to defend?

No, a unit is not considered to be attacking if there are no enemy units present at the battlefield to defend.

According to Rule 437 and Rule 440, the "Attacker" and "Defender" designations only exist during Combat. Combat can only occur between units controlled by opposing players at the same battlefield.

When you move a unit to an empty, uncontrolled battlefield, you initiate a Showdown rather than Combat (Rule 429.1). Because there is no opposing unit present to create a Combat, your unit does not receive the "Attacker" designation. As confirmed in RiftJudge FAQ #5513, FAQ #2505, and FAQ #9883, abilities that trigger "When I attack" will not activate, and passive abilities like Assault will not provide their might bonus because the unit is not an attacker.