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/apitestlib.go

@@ -689,7 +689,7 @@ func (me *TestHelper) GenerateTestEmail() string {
 func (me *TestHelper) CreateGroup() *model.Group {
 	id := model.NewId()
 	group := &model.Group{
-		Name:        "n-" + id,
+		Name:        model.NewString("n-" + id),
 		DisplayName: "dn_" + id,
 		Source:      model.GroupSourceLdap,
 		RemoteId:    "ri_" + id,