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>

store/sqlstore/upgrade.go

@@ -798,6 +798,7 @@ func upgradeDatabaseToVersion524(sqlStore SqlStore) {
 	// if shouldPerformUpgrade(sqlStore, VERSION_5_23_0, VERSION_5_24_0) {
 
 	sqlStore.CreateColumnIfNotExists("UserGroups", "AllowReference", "boolean", "boolean", "0")
+	sqlStore.GetMaster().Exec("UPDATE UserGroups SET Name = null, AllowReference = false")
 	sqlStore.AlterPrimaryKey("Reactions", []string{"PostId", "UserId", "EmojiName"})
 
 	// 	saveSchemaVersion(sqlStore, VERSION_5_24_0)