Otherwise, the empty string is used by Excel, so the field arrives
unnamed, disappearing from pivot table's cells upon refresh, etc.

Change-Id: I2e0f9fad2d766829f2b06425bec24ff182e3a869
Reviewed-on: https://gerrit.libreoffice.org/71065
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>