Does a unit gain the Might Assault bonus on an empty battlefield?

No, a unit does not gain the might bonus from Assault when moving to an empty battlefield.

According to Rule 807.1.c, Assault is functionally short for "While I am an attacker, I have +X Might." Rule 807.1.d clarifies that a unit only gains the "Attacker" designation during Combat.

As established by RiftJudge FAQs (e.g., FAQ #9076, #863), moving a unit into an unoccupied (empty) battlefield initiates a non-combat showdown, not a combat. Because there is no opposing unit present to create combat, your unit does not receive the "Attacker" designation. Since it is not an attacker, the Assault ability remains inactive, and the might bonus is not applied.