Try to avoid an error probably associated with empty package licenses

guix-data-service/model/license-set.scm

@@ -32,5 +32,7 @@
    '(license_ids)
    (vector-map
     (lambda (_ license-ids)
-      (list (sort license-ids <)))
+      (if (= 0 (vector-length license-ids))
+          (list (cons "integer[]" license-ids))
+          (list (sort license-ids <))))
     license-id-lists)))

tests/model-license-set.scm

@@ -17,7 +17,8 @@
        #f)
       ("License 2"
        #f
-       #f))))
+       #f))
+     ()))
 
 (with-postgresql-connection
  "test-model-license-set"