Allow the modification on FcTypeVoid with FcTypeLangSet and FcTypeCharSet

FcTypeVoid is likely to happen when 'lang' and 'charset'
is deleted by 'delete' or 'delete_all' mode in edit.
Without this change, any modification on them are simply
ignored.

This is useful to make a lot of changes, particularly
when one wants to add a few and delete a lot say.