erp5_simulation: Display Divergence dialog even if there is nothing to solve...

erp5_simulation: Display Divergence dialog even if there is nothing to solve (same as legacy Simulation Dialog).

This fixes the following Site Error when there is no Divergence to solve:
  AttributeError: 'NoneType' object has no attribute 'SolverProcess_getSolverDecisionList'

This may happen if user selects 'Solve Divergence' Action and solve
Divergences, then immediately goes back and selects again this action
whereas there is actually no Divergence to solve.

bt5/erp5_simulation/SkinTemplateItem/portal_skins/erp5_simulation/Delivery_getSolverDecisionList.py

 solver_process = context.Delivery_getSolverProcess()
+if solver_process is None:
+  return []
+
 # XXX should omit 'solved' decision?
 return solver_process.SolverProcess_getSolverDecisionList()