Rescissory action against an arbitration award. Administrative contract. Exploitation of piped gas services. Private-law regime. The fact that the matter involves a public tender and that the tender notice does not contain an arbitration clause is not an obstacle to the parties resolving their disputes through arbitration. Validity of the arbitration award. In the absence of an arbitrator appointed by one of the parties, where proper notice has been given, the arbitration award is established by majority decision and there is no nullity whatsoever (Compagás case):