add check for null player name in Team methods

[Doc94]

Close #13467
This PR just mention in docs the behaviour for null player name and make the check in the method before the later check in addEntry.

You can test this with

this.getServer().getCommandMap().register("fallback", new BukkitCommand("test", "cool hi command", "<>", List.of()) {
            @Override
            public boolean execute(@NotNull CommandSender sender, @NotNull String commandLabel, @NotNull String[] args) {
                if (sender instanceof Player player) {
                    Team team = player.getScoreboard().getTeam("DOC");
                    if (team == null) {
                        team = player.getScoreboard().registerNewTeam("DOC");
                    }
                    OfflinePlayer offlinePlayer = Bukkit.getOfflinePlayer(UUID.fromString("00000000-0000-0000-0000-000000000000"));
                    player.sendMessage(Component.text("Offline player: " + offlinePlayer.getName()));
                    team.addPlayer(offlinePlayer);
                }
                return true;
            }
        });