[Dev] Small tweak to parcel loader

[Morgen Sagen]

By popular demand:  if you want to set the defaultValue or initialValue 
of a list or dict attribute to be an empty list or dict, you may now 
include either "<defaultValue/>" or "<initialValue/>" inside that 
attribute's definition.  For example:

   <Attribute itemName="listAttr">
     <displayName>List Attribute</displayName>
     <cardinality>list</cardinality>
     <type itemref="String"/>
     <initialValue/>
   </Attribute>

~morgen