[API] Fix #3203

src/server/api/common/get-host-lower.ts

@@ -1,5 +1,6 @@
 import { toUnicode } from 'punycode';
 
 export default (host: string) => {
+	if (host == null) return null;
 	return toUnicode(host).toLowerCase();
 };

src/server/api/endpoints/users/notes.ts

@@ -28,7 +28,7 @@ export const meta = {
 		},
 
 		host: {
-			validator: $.str.optional,
+			validator: $.str.optional.nullable,
 		},
 
 		includeReplies: {