ERTs occasionally not spawning

### Testmerges

#7850: 'Acid Runner: Acid gen cap' by TheManWithNoHands at commit 267636ef4a #7607: 'Warden Praetorian rework (tm)' by Red-byte3D at commit af402858bf #7650: 'Yet Another Attempt To Fix Vamp' by ihatethisengine at commit eaf54699e6 #8226: 'Project ARES: Turret IFF Controls' by realforest2001 at commit 04e258921d #8026: 'Xenomorph Endgame changes' by Git-Nivrak at commit 3c6c29654b #8277: 'Adds a shotgun choke for the M37A2' by VileBeggar at commit 8d8b7b042c #8270: 'The Mutiny Test' by realforest2001 at commit d460cd3bad #7816: 'Hostile Survivor Rework Revival' by BOBAMAx at commit f5a368b3a7 #8346: 'Netspeak replacer system that automatically converts common internet terms into either emotes or proper language' by micr0-dev at commit 1af34b9fae #8356: 'Fixes CO's not having techweb access / not having the icon' by Red-byte3D at commit 25b9fde880 #8359: 'removes chatsaving chug on 516' by hry-gh at commit a2007936cc #8361: 'adds a reloader to prevent chat from bugging out' by hry-gh at commit 72f50153a1

### Round ID

#25279 and more

### Description of the bug

From time to time, an ERT is enabled but doesn't spawn anybody, no matter how many people roll for it. This seems to apply to every ERT, including Predator Hunting grounds, Forsaken xenos groundside and Distress Beacons.

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

ERTs should always spawn

### How do we reproduce this bug?

1. Enable an ERT
2. Roll for it
3. (from time to time) See that no ERT spawns

Not sure if this is reproducible, but it's been happening several times over the past couple of weeks at least.

### Issue Bingo

- [ ] 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)

Start with the ERT enable, roll, and spawn entry points; the issue names no files or tests, so trace those paths and compare Predator Hunting grounds, Forsaken xenos groundside, and Distress Beacons. Done means each listed ERT consistently spawns a participant after being enabled and rolled, including the intermittent case.

Written by the indexing model from the issue text.