don't try to create a link when the link input field is empty

htdocs/core/tpl/document_actions.tpl.php

@@ -7,10 +7,12 @@ if (GETPOST('sendit') && ! empty($conf->global->MAIN_UPLOAD_DOC)) {
 } elseif (GETPOST('linkit') && ! empty($conf->global->MAIN_UPLOAD_DOC)) {
     if ($object->id) {
         $link = GETPOST('link', 'alpha');
-        if (substr($link, 0, 7) != 'http://' && substr($link, 0, 8) != 'https://') {
-            $link = 'http://' . $link;
+        if ($link) {
+            if (substr($link, 0, 7) != 'http://' && substr($link, 0, 8) != 'https://') {
+                $link = 'http://' . $link;
+            }
+            dol_add_file_process($upload_dir, 0, 1, 'userfile', $link);
         }
-        dol_add_file_process($upload_dir, 0, 1, 'userfile', $link);
     }
 }