At the moment, the helm chart value certificate.issuerName is not wired to any option. In addition the chart is dependent on cert-manager as the Issuer is not optional. This seems like a bug/mistake.
I suggest wiring issuerName to the template "certificate-issuer" and certificates issuer ref.
Similarly allow Cert-manager and Issuer to be optional by adding a toggle like done for clientCertificate/serverCertificate
.Values.certificate.createIssuer: true