refacor: improve cache interface (#3492)

* fix: proper typehint on setScope

* refactor: type hint setKey()

* typehint

caches/NullCache.php

@@ -4,11 +4,11 @@ declare(strict_types=1);
 
 class NullCache implements CacheInterface
 {
-    public function setScope($scope)
+    public function setScope(string $scope): void
     {
     }
 
-    public function setKey($key)
+    public function setKey(array $key): void
     {
     }
 
@@ -16,7 +16,7 @@ class NullCache implements CacheInterface
     {
     }
 
-    public function saveData($data)
+    public function saveData($data): void
     {
     }
 
@@ -25,7 +25,7 @@ class NullCache implements CacheInterface
         return null;
     }
 
-    public function purgeCache($seconds)
+    public function purgeCache(int $seconds): void
     {
     }
 }