Handle mon_null in monster upgrade groups

[dseguin]

Summary

None

Purpose of change

• Fixes mon_null from nested mongroups spawns invisible "human" enemies #61622

Describe the solution

When a monster upgrade selects a group entry of mon_null, spawn nothing (as-in, the original monster disappears).

This behaviour can be adjusted slightly with "despawn_when_null":

"upgrades": {
  "half_life": 1,
  "into_group": "mongroup_with_mon_null",
  "despawn_when_null": true
}

If true, the monster disappears without a trace, otherwise the monster "dies" naturally leaving behind whatever it would if it was killed. (defaults to false)

Describe alternatives you've considered

Originally I interpreted getting mon_null as leaving the monster as-is.

Testing

Added a few assertions to the mongroup_multi_spawn_restrictions test:

./tests/cata_test --rng-seed time mongroup_multi_spawn_restrictions

Also tested in-game using Venera3's branch: https://github.com/Venera3/Cataclysm-DDA/tree/meat_cocoon

Additional context