Improve PHP 7.2 compatibility (#3572)

Fixes #3570. Credit to @danharrin

modules/system/controllers/requestlogs/_referer_field.htm

@@ -1,4 +1,4 @@
-<?php if (count($formModel->referer) > 0): ?>
+<?php if ($formModel->referer && count($formModel->referer) > 0): ?>
     <div class="form-control control-simplelist with-icons">
         <ul>
             <?php foreach ((array) $formModel->referer as $referer): ?>
@@ -8,4 +8,4 @@
     </div>
 <?php else: ?>
     <div class="form-control"><em>There were no detected referers to this URL.</em></div>
-<?php endif ?>
+<?php endif ?>