Do you get +1 from Assault in a non-combat showdown?

No, you do not gain the +1 might bonus from Assault in a non-combat showdown.

According to the rules and official FAQs regarding the Assault keyword:

• Assault is conditional: Assault is functionally shorthand for: "While I am an attacker, I have +X Might."
• Combat designation is required: A unit only gains the "Attacker" designation during Combat.
• Combat vs. Showdown: A combat only occurs when a battlefield is contested by units controlled by different players. If you move a unit to an unoccupied (open) battlefield, that initiates a non-combat showdown. Because there is no opposing unit to create Combat, your unit is not designated as an "Attacker," and the Assault bonus is not applied.

This is consistent with RiftJudge FAQ #8593, which explicitly states: "You do not receive the Assault bonus when attacking an open (unoccupied) battlefield... Since your unit is not designated as an Attacker in a non-combat Showdown, the Assault passive ability does not activate."