bin/map: If outputting to a DB, also create output XML elements for NULL input values. This will help with the transition to using the same XML tree for all rows.

xml_func.py: _label: return None on empty input

mappings/VegX-VegBIEN.stems.csv: Added _collapse around subtrees that need to be removed if they are created around a NULL value

xml_func.py: Added _collapse to collapse a subtree if the "value" element in it is NULL

schemas/vegbien.sql: definedvalue: Made definedvalue nullable so that each row of a datasource can have a uniform structure in VegBIEN, and to support reusing the same XML DOM tree for each row

xpath.py: Added is_xpath()

xml_dom.py: set_value(): If value is None and node is Element, remove value node entirely instead of setting node's value to None

xml_dom.py: Added value_node(). Use new value_node() in value() and set_value(). set_value(): If the node already has a value node, reuse it instead of appending a new value node.

xpath.py: put_obj(): Return the id_attr_node using get_1() because it should only be one node

xml_func.py: _simplifyPath: Also treat the elem as empty if the required node exists but is empty