[Certmanager typo fix] Correctly check `ingressClass`, not `server`

When checking if ingressClass is not set, we were incorrectly checking if server was an empty string rather than ingressClass. This was likely a copy/paste error.

This led to the CertManager Issuer having spec.acme.solvers[0].http01.ingress.class equaling "" instead of $releaseName-nginx if global.ingress.class was not explicitly set in the CR.