Enabling `skip-ca` without certificates make TiProxy unable to connect to PD/TiDB

[djshow832]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Deploy TiProxy with this configuration and without certificates:

security.cluster-tls.skip-ca: true
security.sql-tls.skip-ca: true

2. What did you expect to see? (Required)

TiProxy is up

3. What did you see instead (Required)

TiProxy is down

4. What is your version? (Required)

master

[djshow832]

if !cfg.HasCA() {
	if cfg.SkipCA {
		// still enable TLS without verify server certs
		return &tls.Config{
			InsecureSkipVerify: true,
			MinVersion:         GetMinTLSVer(cfg.MinTLSVersion, lg),
		}, nil
	}
	lg.Info("no CA to verify server connections, disable TLS")
	return nil, nil
}

This doesn't make sense. When TiProxy has no certificates and it also enables skip-ca, TiProxy always enables TLS.