P116 ATHENA (GOG-3020/ENGOT-ov45; EudraCT 2017–004557–17; NCT03522246): a randomised, double-blind, placebo-controlled, phase 3 study of the poly(ADP-Ribose) polymerase (PARP) inhibitor rucaparib + the PD-1 inhibitor nivolumab following frontline platinum-based chemotherapy in ovarian cancer (OC)