Fix PHP Notice.

bibcite_footnotes.module

@@ -76,6 +76,9 @@ function bibcite_footnotes_preprocess_bibcite_footnote_list(&$variables) {
   $dont_show_backlink_text = $config['reference_dont_show_backlink_text'];
   $sort_references_by = $config['reference_sort_by'];
   $citation_tools = new CitationTools();
+  if (empty($footnotes)) {
+    return;
+  }
   foreach ($footnotes as $fn) {
     $item = [
       '#id' => $fn['fn_id'],