Let's not export sheet tab color when it's not set.

COL_AUTO is the default value which is treated as color not being set.  The
old code would export it as a white color which is not correct.

Change-Id: Ib68fccfb228333f2059edc4cfffeed90880ebc60
(cherry picked from commit 3104f95254530c76c3fc0c3b840254912c067ad8)
Reviewed-on: https://gerrit.libreoffice.org/11004
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>