Merge pull request #1245 from discord-php/1239-fix/interactions-messa…

…ge-content-cannot-be-null

fix MessageBuilder to allow null on empty payload with files in multipart

src/Discord/Builders/MessageBuilder.php

@@ -624,12 +624,12 @@ public function toMultipart(bool $payload = true): Multipart
     /**
      * {@inheritDoc}
      */
-    public function jsonSerialize(): array
+    public function jsonSerialize(): ?array
     {
         $empty = true;
 
         if (! empty($this->files)) {
-            $body = [];
+            $body = null;
             $empty = false;
         }