Khalid,
This is a drop-in replacement for Jay's version. I'll pass the enhancement along to Jay.
Install it in a new folder, copy client_secrets.json, oauth2.txt and oauth2service.json from your existing folder to the new folder.
No new projects or authorizations are required.
gam csv users.csv gam update user "~User" Schema.Field multinonempty "~MultiValue1" Schema.Field multinonempty "~MultiValue2"
If columns MultiValue1 or MultiValue2 are empty, they will not be added.
Let me know if this does whjat you want,