Despite being optional as per ECMA-376-1:2016, Excel 2016 seems to
require the presence of "name" attribute in dataField element of
pivot table definition, so make sure to write at least empty string
there.

Change-Id: Iaab5674f86b7dd0b267776678e11af47086635d7
Reviewed-on: https://gerrit.libreoffice.org/70522Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Tested-by: Mike Kaganski <mike.kaganski@collabora.com>