bnc#882631: Do not hide top of the first row when editing text in table cells.

This can happen when proportional line spacing is less than 100%
and "FontIndependentLineSpacing" is set, so just keep the height
of the very first line in case of IsFixedCellHeight() too.

Change-Id: Id231fc1f2373645e5420f877e8c2568ce8a1f099
(cherry picked from commit 9388c45c0263a0090f9cb4be9c42b93b5f7985d0)
Reviewed-on: https://gerrit.libreoffice.org/9891
Reviewed-by: Andras Timar <andras.timar@collabora.com>
Tested-by: Andras Timar <andras.timar@collabora.com>