Skip branch with option=0

[Enchufa2]

Use case: set an optional path and be able to skip it. Solution: skip a branch and continue to the next activity if the option function returns 0. So

t <- create_trajectory() %>%
  branch(function() 0, F, create_trajectory() %>%
           timeout(function() { print("optional"); 0 })) %>%
  timeout(function() { print("main"); 0 })

simmer() %>%
  add_generator("dummy", t, at(0)) %>%
  run()
#> Error: Branch: index out of range

should print main instead of the error.