This fixes a minor bug introduced by
10af8bb24b39a815079f6bf31b449c6e5aaa2adf.
Before the commit, the interface group was set only when Group= is explicitly
specified, otherwise the interface group was kept. However, after the commit,
we need to specify Group= with an empty string to keep the current interface
group.
.required_for_online = -1,
.required_operstate_for_online = LINK_OPERSTATE_RANGE_DEFAULT,
.activation_policy = _ACTIVATION_POLICY_INVALID,
+ .group = -1,
.arp = -1,
.multicast = -1,
.allmulticast = -1,