MFHS performs weirdly on MultiZ

### Testmerges

N/A, multiround.

### Round ID

N/A, multiround.

### Description of the bug

Throwing a MFHS grenade on an upper Z-level leads to the metal foam filling empty spaces. Meaning you can form a wall in mid air.

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

The foam should either go down a Z-level or just not expand past the edge.

### How do we reproduce this bug?

1.Throw MFHS on an upper z-level near a ledge.
2.Watch

### 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)