So, is there a way to modify a photo so that its description is truly empty in the eyes of a Smart Album? And, thank you for your help.
The basic problem here is that deleting a description in Photos sets the description field of the photos database to an empty string ("") rather than an empty field (NULL). I reckon this is a bug in the Photos app. The database can be fixed by resetting all descriptions with empty strings to NULL using an SQL query in the Terminal.
Make sure the Photos app is closed, then go to Terminal and enter the following commands (change path to that of your own Photos Library).
$ sqlite3 ~/Pictures/Photos\ Library.photoslibrary/database/Library.apdb
SQLite version 3.8.10.2 2015-05-20 18:17:19
Enter ".help" for usage hints.
sqlite> update RKVersion set extendedDescription=null where extendedDescription="";
sqlite> .exit
This procedure should be repeated whenever descriptions have been deleted in Photos.
Keep in mind that modifying the Photos database is potentially dangerous, so you might want to make a backup first.