Use eager loaded tag state + null handling

Closes #72, ref #45 and #46.
Seems like the only issue with the original PR is that a null state (doesn't exist) wasn't handled, while the original code always returned a dummy state if it didn't exist in the DB.
The added '?? null' avoids warnings/notices regarding null->subscription (which depends on PHP version)

src/AddTagSubscriptionAttribute.php

@@ -20,9 +20,9 @@ class AddTagSubscriptionAttribute
     public function __invoke(TagSerializer $serializer, Tag $tag, array $attributes): array
     {
         /** @var TagState $state */
-        $state = $tag->stateFor($serializer->getActor());
+        $state = $tag->state;
 
-        $attributes['subscription'] = $state->subscription;
+        $attributes['subscription'] = $state->subscription ?? null;
 
         return $attributes;
     }