Support emptying groups when no members in list and auth_membership is true

[legooolas]

This just allows passing through of the empty to_be_added list to the gpasswd -M arguments list, which causes the group to indeed be emptied successfully.

(Also moved the "Ignoring unknown user" debug print to the rescue block in members_insync?, as this is where the control jumps to on getpwuid or getpwnam throwing an exception when the user isn't found. No logic change, just that the message was printed when users were found rather than not 👍 )

[legooolas]

I need to test this more on different platforms, but it's working as a first-pass change on my Ubuntu 22.04 test box :)

[legooolas]

Fixes #20

[legooolas]

Tested on Ubuntu 18.04/20.04/22.04, CentOS 6, CentOS 7, Rocky 8 and all working OK, so presumably will work on pretty much everything this module already worked on :)

[op-ct]

Thanks, @legooolas!

The code lgtm 👍, but be aware that it might take a while before someone is free to add unit tests for this prior to a release—however, we'd happily accept any tests if they were contributed!