Notes in nested groups break sequence diagrams

[nsw42]

Apols if this is a duplicate - there are some similar looking tickets, such as #2725, but I've identified a minimal repro.

Seen in v0.7.1

An actor using a self message in a nested group works as expected:

shape: sequence_diagram

actor: Actor

outer group: {
  inner group: {
    actor -> actor: "Think about life"
  }
}

and an actor using a self message in a single group also works as expected.

However, a note in place of the self message reports end of JSON on play.d2lang.com, and results in weird SVG (and fails to produce a PNG) using the offline version.

shape: sequence_diagram

actor: Actor

outer group: {
  inner group: {
    actor."Think about life"
  }
}