#61643 [13.x] Use default view when make:notification --markdown has no value

php artisan make:notification InvoicePaid --markdown silently creates a plain notification without a markdown view, because the option is null when passed without a value.

make:mail had the same issue and it was fixed in #39565. This does the same for make:notification, so it falls back to mail.invoice-paid, the same default the interactive prompt already uses