fix(Misc): Remove an annoying warning (#2909112 by e0ipso)

src/Form/JsonapiResourceConfigForm.php

@@ -306,7 +306,7 @@ class JsonapiResourceConfigForm extends EntityForm {
       '#open' => TRUE,
     ];
 
-    $markup .= '';
+    $markup = '';
     $markup .= '<dl>';
     $markup .= '<dt>' . t('Disabled') . '</dt>';
     $markup .= '<dd>' . t('Check this if you want to disable this field completely. Disabling required fields will cause problems when writing to the resource.') . '</dd>';