message: Add explicit operator bool to actor_addr
diff: @@ -78,6 +78,10 @@ public:
  /// Exchange content of `*this` and `other`.
  void swap(actor_addr& other) noexcept;
  + inline explicit operator bool() const {
  + return static_cast<bool>(ptr_);
  + }
  +
  /// @cond PRIVATE
  static intptr_t compare(const actor_control_block* lhs,
  