PLUTO-609: Reimplementation for new interpretation of PLT.17.1 concerning getting and setting of null values for portlet preferences. See: https://issues.apache.org/jira/browse/PLUTO-609?focusedCommentId=13105053#comment-13105053 for details
portals/pluto/trunk/pluto-container/src/main/java/org/apache/pluto/container/impl/PortletPreferencesImpl.java