Can't use empty type in models

The bottom type `Union{}` has some nice properties. For example, a `DiscreteCategory{Ob}` could be a model of `ThCategory{Ob, Union{}}`. However, GATlab tries to instantiate the method `id(x::Union{}) = ...` and the Julia compiler rejects this due to a [breaking change](https://github.com/JuliaLang/julia/issues/52385) in Julia 1.10 that has persisted in 1.11. In Julia 1.12 it might be addressed.

So this issue is just documenting an upstream problem rather than anything to be done in GATlab itself.

In practice, this just leads to definitions like `DiscreteCategory{Ob, Hom}` - we have to manually declare what the homs *would* be when creating a discrete category, which is cumbersome. Another solution is that we can use a type parameter: `@instance ThCategory{Ob, T} where {T<:Union{}}` (the compiler does *not* reject `f(x::T) where {T<:Union{}}`), though this results in a bunch of "`T` was declared but not used" warnings.