Manage edge cases in value functions

[gregorgorjanc]

Following the QG vignette code here is one quirk that we should think how to address in future:

# I am removing all workers and the calcColonyPheno() or the mapping FUN fails
> calcColonyPheno(removeWorkers(colony, p = 1.0), queenTrait = "queenTrait", workersTrait = "workersTrait")
Error in valueFUN(colony@workers) : 
  trying to get slot "pheno" from an object of a basic class ("NULL") with no slots

[janaobsteter]

Do we want to add a check that if there's a workersFUN provided and there are no workers, we throw a message (or a 0)?

[gregorgorjanc]

Yeah, that would be good I think - throw a warning and return 0 for that colony component.

[janaobsteter]

Implemented in #625 - without warning though, but added in the functions description.