Valid Cell value check in class WorkSheetEncoder

```
What steps will reproduce the problem?
1.Create a new cell with value System.DBNull.Value
2.Save Workbook

What is the expected output? What do you see instead?
Expected output is to treat this value the same as null. Instead an exception
"Invalid cell value." is thrown.

What version of the product are you using? On what operating system?
20090331. Windows server 2003.

Please provide any additional information below.
I corrected this with changing line #62 from:
if (cell != Cell.EmptyCell && cell.Value != null)

to:
if (cell != Cell.EmptyCell && cell.Value != null && cell.Value !=
System.DBNull.Value)
```

Original issue reported on code.google.com by `halliday...@gmail.com` on 17 Aug 2010 at 12:05

Start at line 62 of the WorkSheetEncoder class and reproduce the issue by creating a cell with System.DBNull.Value, then saving the workbook. Done means DBNull.Value is treated like null and saving completes without an "Invalid cell value." exception.

Written by the indexing model from the issue text.