doc: Build an empty index if the manual is missing.

That way, the "sk" index for 'guix-manual' is empty, because
'guix-manual' lacks an "sk" translation.

* doc/build.scm (html-manual-identifier-index)[build](html-files):
Gracefully handle 'scandir' returning #f.

1 files changed, 3 insertions, 2 deletions

diff --git a/doc/build.scm b/doc/build.scm
index f5c8713b412..cf3e5d28305 100644
--- a/doc/build.scm
+++ b/doc/build.scm
@@ -309,8 +309,9 @@ actual file name."
             (define (html-files directory)
               ;; Return the list of HTML files under DIRECTORY.
               (map (cut string-append directory "/" <>)
-                   (scandir #$manual (lambda (file)
-                                       (string-suffix? ".html" file)))))
+                   (or (scandir #$manual (lambda (file)
+                                           (string-suffix? ".html" file)))
+                       '())))
 
             (define anchors
               (sort (concatenate