If both sides lose all their units in Invasion Combat, who wins?

Question

Accepted Answer

The defender keeps the planet: if all Ground Forces on both sides die, the rulebook says the defender retains control and places a Control Marker. One reply says a surviving Mercenary counts as a unit, so its owner wins the combat, but a lone Mercenary leaves the planet neutral under the older rule.

Twilight Imperium: Third Edition

If both sides lose all their units in Invasion Combat, who wins?