Ensure key is None to support unencrypted packets

[takeshixx]

In order to support unencrypted packets, the secret has to be None:

https://github.com/ansible/tacacs_plus/blob/de0d01372169c8849fa284d75097e57367c8930f/tacacs_plus/packet.py#L86-L88

However, getpass() will return a empty string, which will not if you intent to generate encrypted packets.

https://github.com/ansible/tacacs_plus/blob/de0d01372169c8849fa284d75097e57367c8930f/bin/tacacs_client#L150-L151

This commit simple sets args.key to None in case no secret has been provided.

[takeshixx]

ping

[crisidev]

Thanks for the contribution and sorry for the long wait. I somehow lost track of the notifications for this repository. This looks good. Merged.