Fix MathML to ACSet pipeline for case with no transitions

[adarshp]

We need to fix the MathML to ACSet pipeline to handle Petri nets without transitions.

@liunelson reports:

When I send the equations d A / d t = 0, it returns a Petri net of the form "0" -> "A" where "0" is a transition node. I'd have expected the correct response should be just a trivial Petri net with just a single species node "A" and nothing else.

[cl4yton]

@cl4yton check with Adarsh about this.

[cl4yton]

Discussed with Adarsh: not super urgent, not required for M8