NPE when retrieving missing r attrs for cell and row in sheetreader

[rvdwenden]

There are xlsx files that have no r attribute for cells (and/or rows). These files are normally not generated by Excel or Libre Office, but by other software. And they can be opened by Excel or Libre Office in a normal way.
But the current version of the excel-streaming-reader cannot cope with these type of xlsx's:
it yields null for element.getAttributeByName(new QName("r")).