agent: Allow NULL for R_PADDING, when calling scd and tpm2d.

* agent/call-scd.c (padding_info_cb): Allow NULL.
(agent_card_pkdecrypt): Likewise.
* agent/divert-scd.c (divert_pkdecrypt): Likewise.
* agent/divert-tpm2.c (divert_tpm2_pkdecrypt): Likewise.

It's for RSA PKCD#1 encoding if the decrypt operation removes padding
or not. When caller knows it's not RSA, this information is no use
and it is better to allow NULL with the variable R_PADDING.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>