Fix performance issue, especially with remote filesystems (#41)

Reduce the number of times the filesystem is accessed

src/Listener/UserCoverRelationship.php

@@ -58,12 +58,11 @@ public function __invoke(UserSerializer $serializer, User $user): array
      */
     public function thumbnailUrl(?string $imageName)
     {
-        $thumbnailName = 'thumbnails/' . $imageName;
-
-        if ($this->coversDir->exists($thumbnailName) && !empty($imageName)) {
-            return $this->coversDir->url($thumbnailName);
+        if (empty($imageName) {
+            return null;
         }
-
-        return null;
+
+        $thumbnailName = 'thumbnails/' . $imageName;
+        return $this->coversDir->url($thumbnailName);
     }
 }