NullPointerException is thrown when importing a file having a protected sheet with empty password

Steps to reproduce

1. load attached file

Actual result

java.lang.NullPointerException
at org.zkoss.poi.xssf.usermodel.XSSFSheet.getHashValue(XSSFSheet.java:3810)
at org.zkoss.zss.range.impl.imexp.ExcelXlsxImporter.importPassword(ExcelXlsxImporter.java:574)
at org.zkoss.zss.range.impl.imexp.AbstractExcelImporter.importSheet(AbstractExcelImporter.java:374)
at org.zkoss.zss.range.impl.imexp.AbstractExcelImporter.imports(AbstractExcelImporter.java:145)

Debug Information

• At XSSFSheet.getHashValue(XSSFSheet.java:3810), sheetProtection.selectAttribute("", "hashValue") returns null.
• Even I set a password for a sheet in Excel 2011, the exception still exists.