Fix crash when saving a document that has not used ThymioBlockly

Created by: cwalther

currentSavedXml in that case was an empty string, and passing that to document.setContent() apparently left the QDomDocument in an invalid state that would later crash MainWindow::saveFile().

Add some more error checking for good measure, although I am not currently aware of any circumstance where the error still occurs.

May fix #449 (closed).