[Bug] `lift_operator` edge case: empty subsystem levels complement

[jack-champagne]

Originally reported by @andgoldschmidt on harmoniqs/PiccoloQuantumObjects.jl#57

function lift_operator(
    operator::AbstractMatrix{T},
    indices::AbstractVector{Int},
    subsystem_levels::AbstractVector{Int},
) where T <: Number
 ...
)

The following line breaks if complement is empty, which happens if there are two subsystems:

    full_operator = kron(operator, [Matrix{T}(I(l)) for l ∈ Lᶜ]...)