MM-25014 - Default Group Name to null, until AllowGroupReference is set (#14651)

* make Group.Name a pointer to allow null

* fix unit tests

* fix build error

* fix unit test

* ensure Name field not nil

Co-authored-by: mattermod <mattermod@users.noreply.github.com>

api4/channel_test.go

@@ -3611,7 +3611,7 @@ func TestGetChannelMemberCountsByGroup(t *testing.T) {
 	id := model.NewId()
 	group := &model.Group{
 		DisplayName: "dn_" + id,
-		Name:        "name" + id,
+		Name:        model.NewString("name" + id),
 		Source:      model.GroupSourceLdap,
 		RemoteId:    model.NewId(),
 	}