Navalis - Larva without Queen/Hivecore can unburrow into the water

### Testmerges

N/A

### Round ID

N/A

### Description of the bug

Title.
Probably more a code fix than a mapping one. But I didn't look that hard.

### What's the difference with what should have happened?

They should unburrow onto the weeds on that z level, not the water.

### How do we reproduce this bug?

1. Late join as burrowed larva without a Queen or Core around
2. Go swimming
3.
...

### Issue Bingo

- [x] Issue could be reproduced at least once
- [x] Issue happened in a recent (less than 7 days ago) round
- [x] Couldn't find an existing issue about this (https://github.com/cmss13-devs/cmss13/issues)