CryptoOutputStream outputs invalid message when used to encrypt empty plaintext

[lavaleri]

In order to perform the "encrypt a zero length plaintext" using a CryptoOutputStream, the correct setup would be to create the CryptoOutputStream, don't write any bytes to it, then immediately call close().

This results in the CryptoOutputStream only writing bytes for an empty final frame (and possibly a footer). It does not write any header bytes, resulting in an invalid message that cannot be parsed by any AWS Encryption SDK implementation.

This should be fixed such that this use case will correctly write header bytes before the message body and footer, resulting in a message that can be successfully decrypted into the plaintext "".