fix:No process left in Model when last pool/lane deleted

[royqh1979]

When there is only one pool in the model, if the pool or its last lane is deleted, its refrencing process is also deleted, and there will be no process left in the model, and no way to create a default one.

And this model can't be saved because it has NO process.

Look the model below, if we remove its pool, save it will get failed(The bpmn file wont get modified).

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