(related: fdo#81033) writerfilter: default tab fill character is space

This avoids an ugly "Character" box in the dialog with a null byte in it.

Change-Id: Iee35a8e1bd6b57fb4eb8ae06722bd79ee1d5d8de
(cherry picked from commit c0b6aadedc9429eee4f4df85957e00e29ccb0c8f)
Reviewed-on: https://gerrit.libreoffice.org/10597
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>