Implode doesnt work

Closes #3294373

src/Form/ConfigLogConfigForm.php

@@ -77,7 +77,7 @@ Examples: <ul>
       '#rows' => 25,
       '#title' => $this->t('Configuration entity names to ignore'),
       '#description' => $description,
-      '#default_value' => implode(PHP_EOL, $config_log_conf->get('log_ignored_config')),
+      '#default_value' => $config_log_conf->get('log_ignored_config') ? implode(PHP_EOL, $config_log_conf->get('log_ignored_config')) : NULL,
       '#size' => 60,
     ];