Add a default workflow template to avoid the error at creation

You get an error, even if you do things in the right way, at workflow creation. The reason is the graph need a Start and an End node, with a transition.

It is really not sweet to get an error alert when you do things in the right way: maybe we can remove the first check, since the workflow is not saved yet ?

Or have a start and end nodes with a default transition each time you create a workflow ? And maybe add a paragraph to explain why it is so important to have a start and end node.

Thanks