Don't add color to the property map if is set to auto.
In this case white color was assumed and tables were
white instead of transparent.

Change-Id: I7f203b8f3831b86ba8de33dc57de227b3029c6d9
Reviewed-on: https://gerrit.libreoffice.org/41255Reviewed-by: Szymon Kłos <szymon.klos@collabora.com>
Tested-by: Szymon Kłos <szymon.klos@collabora.com>