[@types/busboy] FileInfo filename field can be undefined #75400

https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/busboy/index.d.ts#L135

This field can be undefined in practice; as seen here: https://github.com/mscdex/busboy/blob/master/lib/types/multipart.js#L337. The branch returning the file info object can be triggered when the filename is undefined.