schemas/functions.sql: _join*(): Fixed bug where was returning '' instead of NULL when only NULL inputs were provided, because array_to_string() always returns a non-NULL string. Functions must always return NULL in place of '' to ensure that empty strings do not find their way into VegBIEN, and to prevent inconsistencies between row-based and column-based import (row-based import folds empty strings to NULL while column-based import relies on having a clean input table).
schemas/functions.sql: _join*(): Fixed bug where was returning '' instead of NULL when only NULL inputs were provided, because array_to_string() always returns a non-NULL string. Functions must always return NULL in place of '' to ensure that empty strings do not find their way into VegBIEN, and to prevent inconsistencies between row-based and column-based import (row-based import folds empty strings to NULL while column-based import relies on having a clean input table).