PhpUnit fix

 - PHP Deprecated:  str_replace(): Passing null to parameter #2 ($replace) of type array|string is deprecated in /home/runner/work/magento-lts/magento-lts/lib/Varien/Object.php on line 594

Author: sreichel

lib/Varien/Object.php

@@ -591,7 +591,7 @@ public function toString($format = '')
         } else {
             preg_match_all('/\{\{([a-z0-9_]+)\}\}/is', $format, $matches);
             foreach ($matches[1] as $var) {
-                $format = str_replace('{{' . $var . '}}', $this->getData($var), $format);
+                $format = str_replace('{{' . $var . '}}', (string) $this->getData($var), $format);
             }
             $str = $format;
         }