Avoid an error when there are no references to insert

guix-data-service/model/nar.scm

@@ -60,13 +60,7 @@
           '(store_path hash_algorithm hash size system deriver)
           data)))
 
-    (exec-query
-     conn
-     (string-append
-      "
-INSERT INTO nar_references (nar_id, reference)
-VALUES "
-      (string-join
+    (let ((reference-data
            (concatenate
             (map (lambda (nar-id narinfo)
                    (map (lambda (reference)
@@ -77,10 +71,17 @@ VALUES "
                            (quote-string reference)))
                         (narinfo-references narinfo)))
                  nar-ids
-             narinfos))
-       ", ")
+                 narinfos))))
+      (unless (null? reference-data)
+        (exec-query
+         conn
+         (string-append
           "
-ON CONFLICT DO NOTHING"))
+INSERT INTO nar_references (nar_id, reference)
+VALUES "
+          (string-join reference-data ", ")
+          "
+ON CONFLICT DO NOTHING"))))
 
     (exec-query
      conn