`oapply` errors when called on empty wiring diagram.

I think that this code should create a trivial `OpenGraph`.
```
using Catlab.CategoricalAlgebra, Catlab.Programs, Catlab.Graphs, Catlab.WiringDiagrams

const _, OpenGraph = OpenCSetTypes(Graph, :V)

wd = @relation () begin
end

oapply(wd, OpenGraph[])
```

Instead, it throws an error:
```
MethodError: no method matching colimit(::Type{Tuple{Any, Any}}, ::Catlab.CategoricalAlgebra.FreeDiagrams.BasicBipartiteFreeDiagram{Any, Any})
```