fix: always add user to group cache

When there is no user in a group, and we add one, it's not added to the group cache. So consecutive call of addUser() and getUsers() is inconsistent.
Furthermore, the user cache is never null, so this check is unesserary.

Signed-off-by: Hugo Renard <hugo.renard@protonmail.com>

lib/private/Group/Group.php

@@ -184,9 +184,7 @@ public function addUser(IUser $user): void {
 		foreach ($this->backends as $backend) {
 			if ($backend->implementsActions(\OC\Group\Backend::ADD_TO_GROUP)) {
 				$backend->addToGroup($user->getUID(), $this->gid);
-				if ($this->users) {
-					$this->users[$user->getUID()] = $user;
-				}
+				$this->users[$user->getUID()] = $user;
 
 				$this->dispatcher->dispatchTyped(new UserAddedEvent($this, $user));