Does Kha'zix Voidreaver gain XP if there are no enemy units at a battlefield when conquering?

No, Voidreaver (the Kha'Zix Legend) does not gain XP if there are no enemy units present when you conquer a battlefield.

According to RiftJudge FAQ #9092, conquering a battlefield with no enemy units present is a Showdown (or simply establishing control), not a combat. Since Voidreaver’s ability specifically reads, "When you win a combat, gain 1 XP," it will not trigger when you conquer an empty battlefield because no combat occurred.