Improve null handling

guix-data-service/database.scm

@@ -41,6 +41,8 @@
             with-advisory-session-lock/log-time
             obtain-advisory-transaction-lock
 
+            NULL
+            NULL?
             exec-query-with-null-handling))
 
 ;; TODO This isn't exported for some reason
@@ -279,6 +281,10 @@
                 "SELECT pg_advisory_xact_lock($1)"
                 (list lock-number))))
 
+(define NULL (make-symbol "null"))
+
+(define NULL? (lambda (s) (eq? s NULL)))
+
 (define squee/libpq
   (@@ (squee) libpq))
 
@@ -304,7 +310,7 @@
              ((string-null? val)
               (if (eq? 1 (%PQgetisnull
                           (squee/unwrap-result-ptr result-ptr) row-i col-i))
-                  '()
+                  NULL
                   val))
              (else val))))
         cols-range))