guix package: Allow `guix package -u' to fuction as before.

This is a follow up to 6ddf97f81b

* guix/scripts/package.scm (%options) <"-u">: Only check for a flag when
there is an ARG after '-u'.

guix/scripts/package.scm

@@ -486,7 +486,7 @@ (define %options
                              arg-handler))))
          (option '(#\u "upgrade") #f #t
                  (lambda (opt name arg result arg-handler)
-                   (when (string-prefix? "-" arg)
+                   (when (and arg (string-prefix? "-" arg))
                      (warning (G_ "upgrade regexp '~a' looks like a \
 command-line option~%")
                               arg)