fix(diagramlink): populate `positionFrom` and `positionTo`

[TonyMasse]

with {x:0, y:0} in DiagramModel.addLink(). To prevent error message when new link is created. As the link's curve cannot otherwise be drawn properly (as no x nor y yet defined for positionFrom and positionTo) until the first call to updateLinksPositions().

Close #6

What kind of change does this PR introduce? (check at least one)

• [x] Bugfix
• [ ] Code style update
• [ ] Feature
• [ ] Refactor
• [ ] Build-related changes
• [ ] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• [ ] It's submitted to the develop branch (or to a previous version branch), not the master branch
• [x] When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• [x] All tests are passing
• [ ] New/updated tests are included

If adding a new feature, the PR's description includes:

• [ ] A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

Problem was caused by DiagramModel.addLink() that was pushing a Link with empty positionFrom and positionTo.