fastparser: avoid boost::optional where it is un-necessary.

boost::optional appears to show up rather heavily on many profiles.
We already use mnElementToken == DONTKNOW to flag / use these guys.

Change-Id: Ibf2b0167f259cc601da2fb9703e880b78e60886e
Reviewed-on: https://gerrit.libreoffice.org/7424
Reviewed-by: Matúš Kukan <matus.kukan@collabora.com>
Tested-by: Matúš Kukan <matus.kukan@collabora.com>