Say that there are no lint warnings

Rather than having an empty table.

guix-data-service/web/revision/html.scm

@@ -280,7 +280,15 @@
            (th "Message")
            (th "Location")))
          (tbody
-          ,@(map
+          ,@(if (null? lint-warnings)
+                `((tr
+                   (td (@ (colspan 3)
+                          (align "center"))
+                       "No lint warnings "
+                       (span
+                        (@ (class "label label-success"))
+                        "✓"))))
+                (map
                  (match-lambda
                    ((id lint-checker-name lint-checker-description
                         lint-checker-network-dependent
@@ -319,7 +327,7 @@
                                          `(li ,@output)))))
                                    git-repositories)))
                              '())))))
-             lint-warnings)))))))))
+                 lint-warnings))))))))))
 
 (define (view-revision/git-repositories git-repositories-and-branches
                                          commit-hash)