Prevent GSettings from freaking out over a missing junk filter.

Boy does GSettings like to abort applications!  The default value for
the 'junk-default-plugin' key is 'Bogofilter' but if the key's mapping
function cannot find Evolution's Bogofilter integration module (it is
optional after all, so may not be installed), then indicating failure
will make GSettings abort Evolution.  So always indicate success.