split(f, 0) throws BoundsError for zero-leg tree #541

Docstring claims split holds for all M between 0 and N, but for M == 0, coupled₁ = leftunit(f.uncoupled[1]) indexes an empty tuple. No live caller hits M=0 today. Fix: use leftunit(f.coupled) instead (equivalent by unit homogeneity).

src/fusiontrees/basic_manipulations.jl:39.