Nether portals can replace unbreakable blocks (make entrances to the void)

By creating a nether portal in the overworld at y=1 (portal blocks at y=2), a nether portal may generate at y=0 (portal blocks at y=1).

Steps to reproduce (unbreakable blocks):

1. Make a nether portal in the overworld.
2. Clear surrounding blocks in the nether side, and make a floor of bedrock, end portal, end portal frame, barriers, command blocks or structure blocks.
3. Go through the portal in the overworld again.
4. The portal should generate in the unbreakable blocks.

Steps to reproduce (entrance to void):

1. Make a nether portal at y=1 in the overworld.
2. You should get a portal at a different y value that y=0.
3. Clear surrounding blocks all the way down to y=1.
4. Go through the portal in the overworld again.
5. The portal should generate at y=0 now.

Possible fix:
Make nether portals unable to replace bedrock, end portal, end portal frame, barriers, command blocks and structure blocks.