visualize fails if no target is specified for an event

[karfau]

The main case I'm aware of is when an event only triggers actions but doesn't change the state.

{
  "initial": "initial",
  "states": {
    "initial": {
      "on": {
        "LOG": {
          "actions": ["log"]
        }
      }
    }
  }
}

I know the example doesn't make a lot of sense on it's own, but it's a minimal reproduction case.
It is displayed correctly in the interactive viz but fails in this sandbox with latest versions of xstate and xstate-plantuml

I tweaked the code to get some output for a state machine that makes heavy use of it, but I'm not sure the way I have done it considers all cases.