Fix SigmaMigration DiagramHom validation

In https://github.com/AlgebraicJulia/Catlab.jl/pull/969 I removed the validation check on the `DiagramHom` that is constructed by `SigmaMigration`. The main purpose of the data is to give both the result of the $\Sigma$ migration as well as what part is the representable element (e.g. there are three edges in the representable edge of a reflexive graph - one of them is special).

First a minimal example where this fails should be constructed and put in the tests, then the reason the validation is failing should be identified and fixed.