message: Make "PreFnWithDT"'s "next" not "optional"
  fixes typegoose#64
diff: @@ -10,7 +10,7 @@ type NDA<T> = number | DocumentType<T> | DocumentType<T>[];
  type ClassDecorator = (target: any) => void;
  type HookNextErrorFn = (err?: Error) => void;
  -type PreFnWithDT<T> = (this: DocumentType<T>, next?: EmptyVoidFn) => void;
  +type PreFnWithDT<T> = (this: DocumentType<T>, next: EmptyVoidFn) => void;
  type PreFnWithQuery<T> = (
  this: Query<T>,
  next?: (error?: Error) => void,
  