No decision taken by the commission or act performed under authority of the commission, shall be invalid by reason only of a defect in the appointment of a person purporting to be a member of, or the existence of a vacancy on, the commission, or of the fact that a person who was not entitled to sit and act as a member of the commission sat or acted as a member of the commission at the time when the decision was taken or the act was authorized, if the decision was taken or the act was authorized by the requisite majority of the members of the commission who were present at the time and entitled to sit and act as members.