Better handle filtering narinfos

This avoids a query error when trying to insert data for 0 narinfos.

guix-data-service/substitutes.scm

@@ -80,13 +80,7 @@
     (simple-format #t "Got ~A narinfo files\n"
                    (length narinfos))
 
-    (unless (eq? (length narinfos) 0)
-      (with-postgresql-transaction
-       conn
-       (lambda (conn)
-         (record-narinfo-details-and-return-ids
-          conn
-          build-server-id
+    (let ((filtered-narinfos
            (filter-map
             (lambda (narinfo)
               (if (> (narinfo-size narinfo)
@@ -98,4 +92,13 @@
                                    (narinfo-size narinfo))
                     #f)
                   narinfo))
-           narinfos)))))))
+            narinfos)))
+
+      (unless (null? filtered-narinfos)
+        (with-postgresql-transaction
+         conn
+         (lambda (conn)
+           (record-narinfo-details-and-return-ids
+            conn
+            build-server-id
+            filtered-narinfos)))))))