composition: skip empty base layers

[alixander]

if you have this for example, there shouldn't be 1500ms of blank white in the beginning

# d2 example.d2 out.svg --animate-interval=1500

direction: down
steps: {
  1: {
    a -> b
  }
  2: {
    b -> d
    c -> d
  }
  3: {
    d -> e
  }
}

https://discord.com/channels/1039184639652265985/1039185315685998693/1112532294348066937