Issuance transfer should not require asset description

[unsystemizer]

Asset description must be provided in order for asset ownership transfer to work:

$ /usr/local/bin/counterparty-client --config=/home/user/.config/counterparty/client.testnet.conf issuance ... --transfer-destination $DESTINATION --description ""

I'll open another issue in counterparty-lib, but as far as the CLI is concerned I would expect that if no description is provided, existing description is used.