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.