Can Krtek Spawn in empty raids?

So the thing is, i had several empty raids in a row (around 9 or 10, seems like no one was playing at the time) and i found cheese and viper sauce most of the time, and Even a purple, so it has been good so far. But Krtek never spawned in any of those raids.

The first raid of that day that had players in it, had Krtek as well. Of course, i died to a kitted duo. Am i missing something? Is there a minimum number of players required for him to Spawn or am i just really unlucky?