In KMCopyDecryptedCommand, when decrypting a mail fails, it will try to create a new Akonadi::Item with no payload, which fails.

This patch simply checks for the return value of MailCommon::CryptoUtils::decryptMessage() and if no mail is returned, it will use the original mail (same behavior as if an unknown encryption is found in CryptoUtils)

"copy decrypted mail to..." with a mail which can not be decrypted
(I have such mails but I still need to investigate why they can not be decrypted)