For the purposes of the Shard of the Throne Law card (VP that passes to a winner of a combat):
Enemy with the shard has one planet with one ground force and no ships in the system.
I move a ship in with one ground force and commence ground combat (no bombardment). Both ground forces score hits on each other and are destroyed.
We ruled that since it was only ground combat and there was no winner (both destroyed), the shard does not pass.
However I did read this elsewhere:
After a combat ends, the player with one or more ships remaining in the system is the winner of the combat; the other player is the loser of the combat. If neither player has a ship remaining, the combat ends in a draw and there is no winner. • If the winner of the combat has fighters or ground forces in space that exceed his ships’ capacity in the active system, he must destroy the excess units of his choice.
Not sure if that only applies to space combat though - there was only ground combat in this particular battle since the opponent had no ships in the system.