make nodes and links optional

Seems that when a workflow without nodes and links is attempted to open on the canvas it fails. Some minor modifications in the calculations and removing the validation resolves the issue in this MR since it is needed in !467 (closed) . BUT it brings the question of validating the workflows when opening and specifically if a workflow needs to have links and nodes even if empty. From the practical point of view a workflow makes sense only if it has at least one node defined. But from the ewoksweb point of view a user can as well attempt to save a totally empty workflow as we also do in our tests. Seems that this must be allowed and have a warning through validation for that.