When playing Lucian, if playing a unit with 2 might with a weapon attached with 2 might, if there are no defenders, does the unit get +1 assault for Sunken Temple trigger?

No, the unit will not have the bonus from Assault when moving into an unoccupied battlefield.

Explanation

According to RiftJudge FAQ #9128 and #9076, Assault is a passive ability that is only active when a unit has the "Attacker" designation. A unit only gains the "Attacker" designation during a combat showdown, which requires an opposing unit to be present on the battlefield.

Moving a unit to an empty (uncontrolled) battlefield initiates a non-combat showdown, not a combat. Because there is no opposing unit present to defend, your unit does not become an attacker, the Assault ability remains inactive, and the +1 [S] bonus is not applied.

Interaction with Sunken Temple

Regarding Sunken Temple and the "Mighty" (5+ [S]) requirement:

1. Assault Bonus: Since you are moving into an empty battlefield, your unit does not receive the "Attacker" designation, so it does not gain the +1 [S] from Assault.
2. Might Calculation: Your unit's current Might would be 4 (2 base Might + 2 Might from the weapon).
3. Result: Because your unit's Might is 4 and not 5+, it is not considered "Mighty," and you will not be able to trigger the ability on Sunken Temple.

Even if an opposing unit were present (initiating a combat), the Assault bonus and "Attacker" designation are removed during the Combat Cleanup phase, immediately before the "Conquer" step (as clarified in RiftJudge FAQ #94 and #1234). Therefore, even in an actual combat, the unit would lose the Assault bonus before Sunken Temple checks for its "Mighty" status.