Function particle_is_explicit_empty 

pub fn particle_is_explicit_empty(p: &ParticleResult) -> bool

True when a particle counts as “empty explicit content” per §3.4.2.3 clauses 2.1.2 / 2.1.3 / 2.1.4.

compositor: None means the particle is a <xs:group ref="…"> whose referenced model group has not been resolved yet — its emptiness can only be decided after group resolution, so we conservatively treat it as non-empty (the safe default for runtime content-type computation).