message: Fix replication of null links
diff: @@ -1106,8 +1106,9 @@ Obj& Obj::set_null(ColKey col_key, bool is_default)
  ColumnType col_type = col_key.get_type();
  // Links need special handling
  if (col_type == col_type_Link) {
  - return set(col_key, null_key);
  + set(col_key, null_key);
  }
  + else {
  auto attrs = col_key.get_attrs();
  if (REALM_UNLIKELY(!attrs.test(col_attr_Nullable))) {
  throw LogicError(LogicError::column_not_nullable);
  @@ -1145,6 +1146,7 @@ Obj& Obj::set_null(ColKey col_key, bool is_default)
  default:
  break;
  }
  + }
  if (Replication* repl = get_replication())
  repl->set_null(m_table, col_key, m_key, is_default ? _impl::instr_SetDefault : _impl::instr_Set); // Throws
  