Do not give an error when the raw value is None.

Give an empty unicode string as output in that case.  We could return None too, does not really matter for me.

I see this in a client project on Plone 3.  The error may be caused by some whacky stuff we do there, based on a beta release of dexterity 1.0, but this pull request does restore the situation from before commit b0300b7f93d67ee52b4aa341dde09dfff60c2121.

(Note for myself: master does not work on Plone 3, due to changed imports.)