[calligra/krita-iterators2-rempt] krita/image/tiles3: Handle the situation where the ng iterators get an empty stretch

[Boudewijn Rempt]

One other idea I had today was make the createIterator methods in KisPaintDevice return 0 if there's nothing to iterate over. After all, we pass shared pointers to iterators around now, so that makes it already actually necessary for the code to check whether we've got a real iterator.

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Boudewijn Rempt
http://www.valdyas.org, http://www.krita.org, http://www.boudewijnrempt.nl