If I move a unit to an empty battlefield, is my unit considered an "attacking unit"?

No, your unit is not considered an "attacking unit."

According to RiftJudge FAQ #5513 and Rule 437, units are only designated as "attackers" or "defenders" during Combat. When you move a unit to an empty battlefield, you initiate a Showdown (Rule 429.1), not Combat. Because no opposing unit is present to create Combat, your unit does not receive the "Attacker" designation.

Consequently, 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 (as confirmed in FAQ #863 and FAQ #2505).