First to avoid NullPointerException you have to add this
Row.MissingCellPolicy.CREATE_NULL_AS_BLANK
This will create a blank cell instead of giving you NPE then you can check to make sure nothing went wrong just like what @Gagravarr have said.
Cell cell = row.getCell(j, Row.MissingCellPolicy.CREATE_NULL_AS_BLANK);
if (cell == null || cell.getCellTypeEnum() == CellType.BLANK)
// do what you want