This is extracted from #22390.
This patch adds a new `optional` option to the `model` step. This
means if an optional model returns something blank (`nil` or an empty
collection) then the service won’t fail and will execute the next step.
However if a model is properly returned, the step will try to check if
it is valid or not (if it responds to `#invalid?`). If the model isn’t
valid, then the step will fail (so no change here).
050828d1de