Bug - invalid attribute access error when missing a state handler

[GondekNP]

Related to #321 - if we start in a state that doesn't have a valid state handler (as in below, we have no seedling handler), we get an Unable to get value for ValueResolver(Trees.age). While I wouldn't expect the below to be interesting, I would expect it to run and contain only seedlings, however it throws Unable to get value for ValueResolver(Trees.age).

start organism Tree

  age.init = 0 count
  age.step = prior.age + 1 count

  maturity.init = 0 count
  # NO default maturity.step handler!

  # Start in "seedling" - but maturity has no handler for seedling!
  state.init = "seedling"
  # Stay in seedling forever (no transition)
  state.step = "seedling"

  inSeedling.init = 1 count
  inSeedling.step
    :if(current.state == "seedling") = 1 count
    :else = 0 count

  # Handlers only exist for juvenile and adult - NOT seedling
  start state "juvenile"
    maturity.step = current.age / 10 count
  end state

  start state "adult"
    maturity.step = 1 count
  end state

end organism

Intuitively I think this should maybe throw a warning, but you could also imagine plenty of situations where the user want the initial state to be vanilla and do nothing interesting (changing state by some patch-level or unconditional organism logic for eg), then having stateful behaviors trigger once they get to some state. In either case I would expect to run properly

[sampottinger]

I suppose this is a little undefined right now but that makes sense. If the state is not found, it should just return the an empty handler set.