Fix two bugs. First bug was that `Proxify` did not delegate value when original value was not empty string but an empty list or false value. Second one is that `Unproxify` did not copy delegated values when the root template field was an old field instance and did not have recently added properties.
Add `keep empty value` option. If this option is true, then emply property will not be delegated and will be kept as empty.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@27706 20353a03-c40f-0410-a6d1-a30d3c3de9de