Ignore nulls replace nulls with blank string like ""

[dashrath-chauhan]

I came across an issue when reading, issue is there are lots of blank cells in my excel file, and this should not be the show stopper in my case. I am wondering if there could be a method that handles null and replace null with empty string, so that we can avoid null pointer exception if processing on that data.

cell method in AbstractSheetContentsHandler.java could be changed to handle this.