Fix Type error when proxy headers are empty

Closes #3513009

src/EventSubscriber/StageFileProxySubscriber.php

@@ -164,10 +164,11 @@ class StageFileProxySubscriber implements EventSubscriberInterface {
 
       $query = $this->requestStack->getCurrentRequest()->query->all();
       $query_parameters = UrlHelper::filterQueryParameters($query);
+      $proxy_headers = $config->get('proxy_headers') ?? '';
       $options = [
         'verify' => $config->get('verify'),
         'query' => $query_parameters,
-        'headers' => $this->createProxyHeadersArray($config->get('proxy_headers')),
+        'headers' => $this->createProxyHeadersArray($proxy_headers),
       ];
 
       if ($config->get('hotlink')) {