The UI default is in the SwForm ctor, where the content type sets the
link start char style to STR_POOLCHR_TOXJUMP.

Have the same default in the UNO API in case the import filters don't
set it explicitly, since that breaks clickable hyperlinks on PDF export.

This also fixes the OSL_ENSURE() failure for missing character formats
in SwTextINetFormat::GetCharFormat().

Change-Id: Ifff354d56a569614ca7f5a89dd75f41817300078
Reviewed-on: https://gerrit.libreoffice.org/71366Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins