A TLS server is required by the spec to always send a server certificate, unless an anonymous cipher is used.
In libprocess, this certificate is verified to be valid and trusted when the flag LIBPROCESS_VERIFY_CERT is set to true.
However, when an anonymous cipher is used, the server does not present a certificate, meaning the verification step will not happen. If a TLS server would be allowed to use such a cipher, it could trivially sidestep the security provided by certificate verification.
Therefore, we should always reject connections using anonymous ciphers when certificate verification is enabled.