Poll expiry can be null

`expiry` is marked as nullable to support non-expiring polls in the future, but all polls have an expiry currently.

Author: valzargaming

src/Discord/Parts/Channel/Poll.php

@@ -31,7 +31,7 @@
  *
  * @property PollMedia              $question            The question of the poll. Only text is supported.
  * @property PollAnswerRepository   $answers             Each of the answers available in the poll.
- * @property Carbon                 $expiry	             The time when the poll ends.
+ * @property Carbon|null            $expiry	             The time when the poll ends.
  * @property bool                   $allow_multiselect   Whether a user can select multiple answers.
  * @property int                    $layout_type         The layout type of the poll.
  * @property PollResults|null       $results             The results of the poll.
@@ -101,12 +101,16 @@ protected function getQuestionAttribute(): PollMedia
     /**
      * Return the expiry attribute.
      *
-     * @return Carbon
+     * @return Carbon|null
      *
      * @throws \Exception
      */
-    protected function getExpiryAttribute(): Carbon
+    protected function getExpiryAttribute(): ?Carbon
     {
+        if (! isset($this->attributes['expiry'])) {
+            return null;
+        }
+
         return Carbon::parse($this->attributes['expiry']);
     }