TLS: Fix missing optional intermediate cert error

There was a typo in line 418, which should have read `config.tlsCA.length`, but that caused another error because apparently `"tlsCA": "./ca.pem"` kept getting appended to config.json every time I tried to delete that line, and setting the value to `null`, `undefined`, or an empty string didn't work either. This does a proper check for whether or not the optional intermediate certificate exists at the specified path, and TLS now works for me without an intermediate certificate.