-- force output for NOT NULL values
NVL(your_variable, Value_if_null)
-- OR (force outputs for both NOT NULL and NULL values)
NVL2(your_variable, value_if_not_null, value_if_null)
SELECT NVL(NULL, 'N/A') FROM dual;
NVL2( string_to_test, value_if_NOT_null, value_if_null )