modify management API of 'groups set-members' to support zero -member option

[puetz-partec]

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Is your feature request related to a problem? Please describe. It is currently not easily possible to set the number of members in a group to an empty set since 'group set-member' option requires that at least one -member option is present. However it should be valid that a group is completely empty (at least for transition periods).

Describe the solution you'd like modify the interface of 'groups set-member' to accept that,if no -member option is present and interpret that as an empty member list. Currently the API returns error saying it expects at least a -member count of 1.

Describe alternatives you've considered

1. modify the interface of 'groups set-member' to either accept an -clear flag to indicate that the set of members should be empty.
2. add a 'groups clear' sub-command option to achieve the same

Explain any additional use-cases

Additional context My current workaround is to do this in two steps

1. groups set-members -member u_auth
2. groups remove-members -member u_auth not nice, but it works.