Historically, OpenVPN server setup required a file with a prime for the Diffie-Hellman key exchange algorithm. Recent OpenVPN versions support Elliptic Curve Diffie-Hellman (ECDH) and you can set dh none to enable it.

This option will be especially useful for the new TLS "site-to-site" mode where it can save the user setup time.

From a quick test, ECDH doesn't require the certs to use EC — at least OpenVPN doesn't complain on startup and connection gets up. We can probably just relax the validation and default to dh none if it's not set.