OpenXML Filter: java.lang.NullPointerException occurs on extraction when a cell does not have a value

java.lang.NullPointerException
	at net.sf.okapi.filters.openxml.Cells$Default.addAndUpdate(Cells.java:93)
	at net.sf.okapi.filters.openxml.Cells$Default.add(Cells.java:77)
	at net.sf.okapi.filters.openxml.Cells$Default.add(Cells.java:108)
	at net.sf.okapi.filters.openxml.Worksheet$Default.readWith(Worksheet.java:150)
	at net.sf.okapi.filters.openxml.ExcelDocument.nextPart(ExcelDocument.java:472)
	at net.sf.okapi.filters.openxml.ExcelDocument.nextPart(ExcelDocument.java:493)
	at net.sf.okapi.filters.openxml.Document$General.nextPart(Document.java:277)
	at net.sf.okapi.filters.openxml.OpenXMLFilter.nextInDocument(OpenXMLFilter.java:455)
	at net.sf.okapi.filters.openxml.OpenXMLFilter.next(OpenXMLFilter.java:261)
	at net.sf.okapi.filters.openxml.OpenXMLFilter.next(OpenXMLFilter.java:270)

An example document - cell-with-omitted-value.xlsx