net: validate player_name in register RPC

ruf-der-pilze/scripts/network_manager.gd

@@ -77,6 +77,9 @@ func register(player_name: String, role: String) -> void:
 	if not multiplayer.is_server():
 		return
 	var sender_id := multiplayer.get_remote_sender_id()
+	if player_name.strip_edges().is_empty():
+		kick.rpc_id(sender_id, "Spielername darf nicht leer sein.")
+		return
 	if role == "dm":
 		for p in players.values():
 			if p.role == "dm":