Fix ETag cache PHPStan return type

Author: GrahamCampbell

src/AbstractCachePlugin.php

@@ -209,7 +209,7 @@ private function calculateCacheItemExpiresAfter(?int $maxAge): ?int
      *
      * @return int|null Unix system time. A null value means that the response expires when the cache item expires
      */
-    protected function calculateResponseExpiresAt(?int $maxAge): ?int
+    protected function calculateResponseExpiresAt(?int $maxAge)
     {
         if (null === $maxAge) {
             return null;

src/EtagCachePlugin.php

@@ -41,7 +41,7 @@ public static function serverCache(CacheItemPoolInterface $pool, StreamFactoryIn
         return new self($pool, $streamFactory, $config);
     }
 
-    protected function calculateResponseExpiresAt(?int $maxAge): ?int
+    protected function calculateResponseExpiresAt(?int $maxAge)
     {
         return 0;
     }