disallow using null in users list

[vrozov]

It is possible to bring ES completely unoperational by using null in the list of users in rolesmapping API. Other API maybe subject of the same issue.

[davidlago]

We are doing some "spring cleaning in the fall", and to make sure we focus our energies on the right issues and we get a better picture of the state of the repo, we are closing all issues that we are carrying over from the ODFE era (ODFE is no longer supported/maintained, see post here).

If you believe this issue should still be considered for current versions of OpenSearch, apologies! Please let us know by re-opening it.

Thanks!