multi_associator returns scalar zero instead of vector on early-exit #540

Docstring says the coefficient is a scalar for MultiplicityFreeFusion and a vector for GenericFusion; the success path returns a vector but the early-exit return zero(sectorscalartype(...)) always returns a scalar. Currently latent (only reached via a UniqueFusion caller); will break/be type-unstable for a future GenericFusion use.

src/fusiontrees/basic_manipulations.jl:141-142.