dump null is a way to check data integraty. so u need to run on ur data which u'd like to backup. and it'll affect ur performance of production. so recommend u do it some time idle?
dump tape is a way to check if there's any issue on tapes. syntax shows in this link:
https://kb.netapp.com/support/s/article/what-is-the-command-syntax-for-dump
below are some links u might think useful
https://kb.netapp.com/support/s/article/top-10-ndmp-issues-and-solutions?language=en_US
hopefully helps
Jeff