profiles: Search for ghc conf files only if package db exists.

This avoids having 'find-files' report warnings about searching in
non-existent directories.

* guix/profiles.scm (ghc-package-cache-file)[conf-files]: Only search
  for *.conf files if the search directory exists.

guix/profiles.scm

@@ -500,7 +500,10 @@ (define db-dir
           (string-append #$output "/" db-subdir))
 
         (define (conf-files top)
-          (find-files (string-append top "/" db-subdir) "\\.conf$"))
+          (let ((db (string-append top "/" db-subdir)))
+            (if (file-exists? db)
+                (find-files db "\\.conf$")
+                '())))
 
         (define (copy-conf-file conf)
           (let ((base (basename conf)))