GetMemberships with both keys only filtered groupName and could return
other members.
Reproduce with a non-member and a group that has members:
varlinkctl call --more SOCKET io.systemd.UserDatabase.GetMemberships
'{"userName":"nonmember","groupName":"GROUP","service":"io.systemd.Multiplexer"}'
(cherry picked from commit 2126abeb6d4905f95b963c7f5c5bc4bd92444ba5)