[FIX]Fixed code for export xls null values should not shown in spreadsheet.
[FIX]Fixed code for export xls null values should not shown in spreadsheet.

havt/openerp-web,havt/openerp-web,havt/openerp-web,havt/openerp-web,havt/openerp-web,akretion/openerp-web,akretion/openerp-web,akretion/openerp-web