Default diagram has no linting error

[nazlikaya]

Currently, the default diagram, i.e. a single start event, is marked as invalid as an end event is expected. This means if a user creates a new diagram, they directly see an error message. In order to avoid this situation, we could adjust the default diagram to contain an end event but this would make it difficult for the user to add new elements between start and end event. Therefore, I propose that we allow a diagram without an end event.