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)

• Bugfix
• Code style update
• Feature
• Refactor
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Does this PR introduce a breaking change? (check one)

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• No

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The PR fulfills these requirements:

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• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing
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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.