PKI provider: allow no root issuer is defined to accept only self-signed issued certificates
Description
The default PKI provider creation through SOPC_PKIProviderStack_CreateFromPaths
was failing when no root issuer (nor CRL) was defined. Since it is possible to use a PKI with only self-signed issued certificate without issuer, it should be allowed.
Note: even if it should be allowed, this is not a recommended usage since there is no way to revoke a self-signed certificate (no CRL associated to an issuer).
Modification
Allow creation of PKI with SOPC_PKIProviderStack_CreateFromPaths
with only issued certificates and no issuer but display a warning in log traces to inform there is no way to revoke such certificate.
Fix certificate validation which was failing when no issuer exists before checking if the certificate was self-signed