Nominally this is optional, but RFC 5035 says:

"The encoding of the ESSCertIDv2 for this certificate SHOULD include the
issuerSerial field."

So do write it, it fixes a warning issued by the PAdES validator from
<https://github.com/esig/dss>.

Change-Id: I344f79e17febe82a697a0936a837c17aefa242df