Broken Nether Portals are Used as Valid Exits

The game treats nether portals that contain no minecraft:nether_portals as valid portals to exit from a portal teleport and thus entering a nether portal can result in landing in the nether with no way back.

Steps to reproduce:

1. Create a nether portal and enter it.

2. In creative mode, break a minecraft:nether_portal in the new portal.

3. Return the original portal (suggested method: /kill @s)

4. Enter the original portal.