LP1766982 - CSV downloads contain the string 'null' for null values.

LP1766982 - CSV downloads contain the string 'null' for null values.

Adds a check for null and undefined in the getItemTextContent function
in grid.js which is called by both the code to download grid data as
CSV and for printing grids.

To test
1. Place holds on a couple of titles, one of which has no author. In the
   test data TCN 226, The Crow, has no author data.
2. Pull up the Holds Pull List.
3. Download Full CSV - to download the CSV file and view it in a
   spreadsheet or text editor looking specifically at the Author column.
4. Print Full Grid - to view the data in a printout, since this function
   also affects printing.
5. Go back to the Holds Pull List and click on one of the barcodes under
   the Current Copy column.
6. Click on the List View.
7. Download Full CSV - open the resulting file to inspect the
   Alert Message data.
8. Apply patch and repeat steps 2-7.

Signed-off-by: gcollum <gcollum@gmail.com>
Signed-off-by: Josh Stompro <stompro@stompro.org>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>