Zombified piglins do not spawn in groups

Zombified piglins should spawn in groups of 2-4 according to the herd sizes given in the vanilla behavior pack spawn_rules\zombie_pigman.json. However, they only ever spawn one-at-a-time. The cause seems to be the same as for magma cubes reported at MCPE-138138: their hard-coded spawn rules cancel spawns that would collide with existing entities, and in Bedrock Edition herds attempt to spawn all at the exact same spot (MCPE-66319).

Steps to reproduce

1. Create a large platform in a biome that spawns zombified piglins.

2. Watch.

3. Run /kill @e and keep watching.

Expected result

Zombified piglins spawn in groups of 2-4.

Actual result

Zombified piglins always spawn alone.