DECODE function returns wrong results when passed NULL

The DECODE function was implemented for Oracle compatibility. It works similar to CASE expression, but it is supposed to have one major difference: NULL == NULL
https://docs.oracle.com/database/121/SQLRF/functions057.htm#SQLRF00631

The Spark implementation does not observe this however:

select decode(null, 6, 'Spark', NULL, 'SQL', 4, 'rocks');

NULL

 The result is supposed to be 'SQL'