In any suit, action, or proceeding involving the validity or enforcement of, or relating to, any contract of the joint public agency, the joint public agency shall be conclusively deemed to have been established, except as against the state, in accordance with the Joint Public Agency Act upon proof of the filing of the certificate of creation by the Secretary of State. A copy of the certificate or amended certificate, duly certified by the Secretary of State, shall be admissible in evidence in any suit, action, or proceeding and shall be conclusive proof of the filing and contents thereof.