fix: correctly render note text

Fix a regression from #8787 that was previously fixed in #8440.

packages/backend/src/remote/activitypub/models/note.ts

@@ -200,7 +200,7 @@ export async function createNote(value: string | IObject, resolver?: Resolver, s
 	let text: string | null = null;
 	if (note.source?.mediaType === 'text/x.misskeymarkdown' && typeof note.source?.content === 'string') {
 		text = note.source.content;
-	} else if (typeof note._misskey_content === 'string') {
+	} else if (typeof note._misskey_content !== 'undefined') {
 		text = note._misskey_content;
 	} else if (typeof note.content === 'string') {
 		text = htmlToMfm(note.content, note.tag);