Remove constructor's project id, key file requirements

[dpkirchner]

The Storage class from @googleapis/nodejs-storage doesn't require the caller to pass a project id or keyfile. If you don't specify them, the library will try to look up the values using Google's "application default credentials" code. As such, the throws here can be removed, and the object passed to Storage can be empty or absent:

https://github.com/ARozar/multer-google-storage/blob/master/src/index.ts#L39

See: https://googleapis.dev/nodejs/storage/latest/Storage.html#Storage-examples and https://cloud.google.com/docs/authentication/production#finding_credentials_automatically