Fix warning in SearchIndexSubscriberTest (see #1991)

Description
-----------

| Q                | A
| -----------------| ---
| Fixed issues     | -
| Docs PR or issue | -

Fixes the following warning that currently appears when running any test:

```
Warning: array_reverse() expects parameter 1 to be array, null given in vendor\contao\contao\vendor\symfony\http-client\Exception\HttpExceptionTrait.php on line 34
```

Commits
-------

f146d94 fix warning in SearchIndexSubscriberTest
e860d91 Merge branch '4.9' into fix/search-index-subscriber-test-warning

core-bundle/tests/Crawl/Escargot/Subscriber/SearchIndexSubscriberTest.php

@@ -470,6 +470,12 @@ private function getResponse(bool $asHtml, int $statusCode = 200): ResponseInter
             ->willReturn($statusCode)
         ;
 
+        $response
+            ->method('getInfo')
+            ->with('response_headers')
+            ->willReturn([])
+        ;
+
         return $response;
     }