Does Assault trigger if no Defender?

No — with no enemy units present there's no Combat Showdown, so no Attacker designation is ever granted and Assault never applies.

Assault isn't actually a triggered ability — it's passive "+X [M] while I am an attacker" (Rule 807.1.c), and "attacker" means you hold the Attacker designation. That designation only gets handed out when a Combat opens at a contested battlefield with opposing units present (Rules 466.2.c, 807.1.d). Running a unit onto an empty or uncontested battlefield that you don't control is a Non-Combat Showdown — no designations, no Assault bonus, no Mighty checks off it. If a defender is present, Combat opens, both designations go out, and Assault applies from that point until Combat ends (807.1.d.1).