Don't report a spurious error if no SCEP pkiMessage is ready yet
On a brand new request in the state op_pkcsreq there will be no
pkiMessage to send yet because there is no CSR yet.
It correctly detects this state but also displays the message:
Error reading request. Expected PKCS7 data containing a
PKCSReq pkiMessage, got nothing.
This is confusing if the request eventually succeeds.
It really only needs to report this if it is passed in a file name
to read the message from, otherwise silently return
The same issue existed in the op_get_cert_initial state.