Fix for PHP 8 compatibility.

Closes #3305141

pwa.module

@@ -247,6 +247,9 @@ function _pwa_fetch_offline_page_resources($pages) {
     $response = drupal_http_request(url($page, ['absolute' => TRUE]));
 
     $dom = new DOMDocument();
+    if (empty($response->data)) {
+      continue;
+    }
     @$dom->loadHTML($response->data);
 
     $xpath = new DOMXPath($dom);