Workaround for Facebook data store API behavior regression, fixes saving

Workaround for Facebook data store API behavior regression, fixes saving
of empty notice prefix text in facebook settings.

Filed bug upstream at http://bugs.developers.facebook.com/show_bug.cgi?id=7110

Per documentation, saving a pref value of "" or "0" will delete the pref key:
http://wiki.developers.facebook.com/index.php/Data.setUserPreference

which used to do what we want... Now Facebook throws back an error
"Parameter value is required" when we do this. Workaround appends a
space to empty string or "0" at save time, then trims the string when
we load it.

The input string was already trimmed at pref save time, so this won't
alter any user-visible behavior.

Thanks to ^demon in #mediawiki for pointing out the behavior regression
after testing the identi.ca Facebook app!