schemas/functions.sql, py_functions.sql: Added schema comment that functions must always return NULL in place of the empty string, to ensure that empty strings do not find their way into VegBIEN. Note that row-based import automatically removes empty strings because the intermediate values are stored in XML and our XML DOM traversing code auto-replaces the empty string with NULL. Column-based import, on the other hand, does not, because the intermediate data is stored in database temp tables instead of a DOM tree.