fix($node/s3): #28 - Check `null` of expected sizes
fix($node/s3): #28 - Check `null` of expected sizes

Instead of checking for the existence of a variable name.

[Suggestion by maintainer](https://github.com/Widen/fine-uploader-server/issues/28#issuecomment-37834660)

fixes #28

paul-herrmann/server-examples,FineUploader/server-examples,FineUploader/server-examples,paul-herrmann/server-examples,paul-herrmann/server-examples,FineUploader/server-examples,paul-herrmann/server-examples,FineUploader/server-examples,paul-herrmann/server-examples,FineUploader/server-examples,paul-herrmann/server-examples,paul-herrmann/server-examples,paul-herrmann/server-examples,FineUploader/server-examples,FineUploader/server-examples,FineUploader/server-examples