When performing signed, symmetric encryption in GPGME via gpgme_op_encrypt_sign
the operation fails with GPG_ERR_INV_VALUE, as it expects there to be recipients
present.

gpgme_op_encrypt works as expected, as it accounts for the possibility of no
recipients being passed.

The attached patch changes gpgme_op_encrypt_sign to mimic the behavior of
gpgme_op_encrypt in relation to dealing with a NULL or empty recipient(s).