Fix bug

src/misc/convert-host.ts

@@ -19,3 +19,8 @@ export function extractDbHost(uri: string) {
 export function toPuny(host: string) {
 	return toASCII(host.toLowerCase());
 }
+
+export function toPunyNullable(host: string | null | undefined): string | null {
+	if (host == null) return null;
+	return toASCII(host.toLowerCase());
+}