APCU: use ttl of 0 when passed null (#250)

* APCU: use ttl of 0 when passed null

Under PHP 8.1 alpha builds, a deprecation warning about invalid `$ttl` is emitted when `null` is passed:

```
PHP Deprecated:  apcu_store(): Passing null to parameter #3 ($ttl) of type int is deprecated in /var/www/html/vendor/cache/apcu-adapter/ApcuCachePool.php on line 88
```

This change fixes the issue.

* Remove trailing whitespace from GH editor

Author: Firehed

ApcuCachePool.php

@@ -85,6 +85,10 @@ protected function storeItemInCache(PhpCacheItem $item, $ttl)
             return false;
         }
 
+        if ($ttl === null) {
+            $ttl = 0;
+        }
+
         return apcu_store($item->getKey(), serialize([$item->get(), $item->getTags(), $item->getExpirationTimestamp()]), $ttl);
     }