message: Implicitly construct TableRef/ConstTableRef from nullptr
diff: @@ -34,6 +34,7 @@ public:
  {
  }
  ConstTableRef(const TableRef& other);
  + ConstTableRef(std::nullptr_t) {}
  const Table* operator->() const;
  const Table& operator*() const
  @@ -83,6 +84,7 @@ public:
  : ConstTableRef(t_ptr)
  {
  }
  + TableRef(std::nullptr_t) {}
  TableRef()
  : ConstTableRef()
  {
  