SELECT CASE WHEN field IS NULL THEN 'Empty' ELSE field END AS field_alias
Или по-идиоматично:
SELECT coalesce(field, 'Empty') AS field_alias
SELECT CASE WHEN field IS NULL THEN 'Empty' ELSE field END AS field_alias
Или по-идиоматично:
SELECT coalesce(field, 'Empty') AS field_alias