Model.statistics.solutions collects `nothing` -type solutions

[jardinetsouffleton]

To reproduce:

• Go to the job shop scheduling notebook in SeaPearlZoo.jl
• Run all cells
• After the following cell:

  my_heuristic(x::SeaPearl.IntVar; cpmodel=nothing) = minimum(x.domain)
  SeaPearl.solve!(model; variableHeuristic=SeaPearl.MinDomainVariableSelection{true}(), valueSelection=SeaPearl.BasicHeuristic(my_heuristic))

  Run this:

  counter = Dict{String, Int}()
  for solution in model.statistics.solutions
  if isnothing(solution)
      if haskey(counter, "null")
          counter["null"] += 1
      else
          counter["null"] = 1
      end
  else
      if haskey(counter, "not_null")
          counter["not_null"] += 1
      else
          counter["not_null"] = 1
      end
  end
  end
  counter

You should see a very large (~130k) number of nothing in the model.statistics.solutions