The result was that there were no new domination markers for the round. The rules say the player placing the most new domination markers (ties go to earlier in the round) gets a bonus card.
We ruled that you had to at least put out one new marker, or you didn't count as placing a marker at all, so no bonus card was awarded.
You were correct! The rules [2.2.5] say (my bold):
The player with the most Os in play (minimum of one) ... draws a free card.
In true AoR style this is not, of course, in the Expansion Phase rule section!