Say I'm attempting to conquer an open battlefield with Rek'Sai, while he is going to the battlefield does he still get the plus might as an "attacker" even if there's no units there to attack?

No, Rek'Sai, Breacher does not get the +1 Might from Assault when moving to a battlefield with no enemy units present.

Assault reads "+1 Might while I'm an attacker" (Rule 807.1.c), and a unit only gains the Attacker designation during Combat (Rule 807.1.d). Moving to an empty battlefield creates a non-combat Showdown — there are no opposing units, so no Combat occurs, and your unit never becomes an attacker (Rule 446.1). Without that designation, Assault remains inactive.

The result is the same whether you're conquering an open battlefield or sending Rek'Sai alone to an uncontrolled one — no Combat means no Assault boost.