bnc#592918: Display data description in pivot table.

When there are no column fields and exactly one data field.

Change-Id: I10d2af7e626f98a4d8a981e078e99e7da9207d1c
(cherry picked from commit 902564bfe52e7699cefc80b3334c2eb0a8cacdcf)
Reviewed-on: https://gerrit.libreoffice.org/11493
Reviewed-by: Andras Timar <andras.timar@collabora.com>
Tested-by: Andras Timar <andras.timar@collabora.com>