Always return location IDs from SwitchableHttpCachePurger::purge

eZ Publish Legacy expects that `content/cache` events always return location
IDs they were given (https://github.com/ezsystems/ezpublish-legacy/blob/master/kernel/classes/ezcontentcachemanager.php#L772).

In eZ Publish kernel 6.x, there was an InstantCachePurger that did this
(https://github.com/ezsystems/ezpublish-kernel/blob/6.13/eZ/Publish/Core/MVC/Symfony/Cache/Http/InstantCachePurger.php#L61-L66),
but in 7.0 (and presumably with 6.x with new HttpCacheBundle turned on),
this does not happen.

So here, we make sure that location IDs are always returned, no matter what.

bundle/Cache/SwitchableHttpCachePurger.php

@@ -32,7 +32,9 @@ public function purge($locationIds)
             return $locationIds;
         }
 
-        return $this->purgeClient->purge($locationIds);
+        $this->purgeClient->purge($locationIds);
+
+        return $locationIds;
     }
 
     public function purgeAll()