fix: send null for empty edited_at in mastodon api

packages/backend/src/server/api/mastodon/converters.ts

@@ -279,7 +279,8 @@ export class MastoConverters {
 			emoji_reactions: status.emoji_reactions,
 			bookmarked: false,
 			quote: isQuote ? await this.convertReblog(status.reblog) : false,
-			edited_at: note.updatedAt?.toISOString(),
+			// optional chaining cannot be used, as it evaluates to undefined, not null
+			edited_at: note.updatedAt ? note.updatedAt.toISOString() : null,
 		});
 	}
 }