MDL-73128 badges: Fix warning with external badges

badges/renderer.php

@@ -59,7 +59,8 @@ class core_badges_renderer extends plugin_renderer_base {
 
             $name = html_writer::tag('span', $bname, array('class' => 'badge-name'));
 
-            $image = html_writer::empty_tag('img', ['src' => $imageurl, 'class' => 'badge-image', 'alt' => $badge->imagecaption]);
+            $imagecaption = $badge->imagecaption ?? '';
+            $image = html_writer::empty_tag('img', ['src' => $imageurl, 'class' => 'badge-image', 'alt' => $imagecaption]);
             if (!empty($badge->dateexpire) && $badge->dateexpire < time()) {
                 $image .= $this->output->pix_icon('i/expired',
                         get_string('expireddate', 'badges', userdate($badge->dateexpire)),