Also, resources such as http://use-the-index-luke.com/blog/2014-11/seven-surprising-findings-about-DB2#blog-db2intro-nulls sya the exact opposite:

In other words, ORDER BY col ASC NULLS FIRST is valid, but it doesn’t change the result—NULLS FIRST is anyways the default. Same is true for ORDER BY col DESC NULLS LAST—accepted, but doesn’t change anything. The other two combinations are not valid at all and yield a syntax error.