Do not leave python DataObjectHandleBase in an invalid state

If one tries to assign only a 'path' to the DataObjectHandleBase, provide some reasonable defaults for the other fields instead of leaving the handle silently in an invalid state where any subsequent assignment causes a crash because str (used during the assignment) will try to use uninitialized attributes, resulting in the confusing message that configurables do not have the property that's being assigned to (because setattr fails, and the caller assumes that this because the attribute isn't there when in reality the assignment to the attribute actually raises the exception...)