Analysis Of A Safety Run-In Cohort From Niveau, A Phase 3 Study For Patients With Aggressive Non-Hodgkin Lymphoma In First Relapse Or Progression Not Eligible For High-Dose Chemotherapy (Hdt), Testing Nivolumab In Combination With Gemcitabine, Oxaliplatin (Gemox) Plus Rituximab (R) In Case Of B-Cell Lymphoma