Fixed not null constraint

ref #56

src/models/entities/Runner.ts

@@ -18,7 +18,7 @@ export class Runner extends Participant {
    * Can be a runner team or organisation.
    */
   @IsNotEmpty()
-  @ManyToOne(() => RunnerGroup, group => group.runners, { nullable: false })
+  @ManyToOne(() => RunnerGroup, group => group.runners)
   group: RunnerGroup;
 
   /**