[Feature:System] Don't set given names to empty string (#37)

Currently this script is setting all users preferred names to an empty
string upon running, which causes many database queries to become
substantially longer to account for both values indicating no desired
preferred name.

student_auto_feed/ssaf_sql.php

@@ -49,15 +49,15 @@ class sql {
         user_familyname,
         user_preferred_givenname,
         user_email
-    ) VALUES ($1, $2, $3, $4, $5, $6)
+    ) VALUES ($1, $2, $3, $4, NULLIF($5,''), $6)
     ON CONFLICT (user_id) DO UPDATE
     SET user_numeric_id=EXCLUDED.user_numeric_id,
         user_givenname=EXCLUDED.user_givenname,
         user_familyname=EXCLUDED.user_familyname,
         user_preferred_givenname=
             CASE WHEN users.user_updated=FALSE
                 AND users.instructor_updated=FALSE
-                AND COALESCE(users.user_preferred_givenname, '')=''
+                AND users.user_preferred_givenname IS NULL
             THEN EXCLUDED.user_preferred_givenname
             ELSE users.user_preferred_givenname
             END,