#141 ZK Spreadsheet: font color not saved on file export

[Sam Chuang]

Steps:
1. change font color of a cell in the browser, and export the spreadsheet to a file
2. open the file in MS Excel, font color is not saved

The bug is in org.zkoss.zss.model.impl.ExcelExporter.exportCellFont(Format), and here might be the fix:

if(null!=zkFormat.getFontColor())
return wf;
==>
if(null==zkFormat.getFontColor())
return wf;