always adjust catalogs configuration elements

Pathnames in the list need to be made absolute
"file://" URLs, even if the list has no #f element
to splice the default catalog.

(cherry picked from commit 78e473f017)

racket/collects/pkg/private/config.rkt

@@ -67,7 +67,6 @@
    [else
     (match k
       ['catalogs
-       (if (member #f v)
        ;; Replace #f with default URLs, relative path
        ;; with absolute path:
        (apply append (for/list ([i (in-list v)])
@@ -82,8 +81,7 @@
                           (url->string
                            (path->url
                             (simple-form-path
-                                 (path->complete-path i (path->complete-path (pkg-dir #t)))))))])))
-           v)]
+                             (path->complete-path i (path->complete-path (pkg-dir #t)))))))])))]
       [_ v])]))
 
 (define (update-pkg-cfg! key val)