Mobs don't use nether portals when there is no portal in the nether

The bug

If there isn't a nether portal in the nether, but mobs do try to go to the nether, they will get deleted.

To reproduce

1. Create a new world
2. Create a nether portal, but don't go through it
3. Push a villager into the portal
4. Observe the villager staying in the overworld
5. Go through the portal and back
6. Push a villager into the portal
7. Observe it go through