import: github: Fix incorrect no-release case.

This is a followup to 81c3dc3224.

Since that commit, when /releases returned an empty JSON array, we would
not fall back to /tags because of the incorrect match.

* guix/import/github.scm (fetch-releases-or-tags): Match the empty
vector instead of the empty list.

guix/import/github.scm

@@ -161,7 +161,7 @@ empty list."
         url))
 
   (match (json-fetch (decorate release-url) #:headers headers)
-    (()
+    (#()
      ;; We got the empty list, presumably because the user didn't use GitHub's
      ;; "release" mechanism, but hopefully they did use Git tags.
      (json-fetch (decorate tag-url) #:headers headers))