Added fix for null value cell.v (#1730)

Closes #1717

Author: geoffrymichael

bits/81_writeods.js

@@ -78,7 +78,7 @@ var write_content_ods/*:{(wb:any, opts:any):string}*/ = (function() {
 						ct['office:value'] = (cell.v||0);
 						break;
 					case 's': case 'str':
-						textp = cell.v;
+						textp = cell.v == null ? "" : cell.v;
 						ct['office:value-type'] = "string";
 						break;
 					case 'd':