[binary_sensor] Fix reporting of 'unknown' (#12296)

Co-authored-by: Jonathan Swoboda <154711427+swoboda1337@users.noreply.github.com>
Co-authored-by: J. Nick Koston <nick@home-assistant.io>

esphome/components/binary_sensor/binary_sensor.h

@@ -63,6 +63,8 @@ class BinarySensor : public StatefulEntityBase<bool>, public EntityBase_DeviceCl
 
  protected:
   Filter *filter_list_{nullptr};
+
+  bool set_new_state(const optional<bool> &new_state) override;
 };
 
 class BinarySensorInitiallyOff : public BinarySensor {