An https.request with servername: "" cleared the TLS hostname, which
also turned off the certificate's name check, so a certificate for any
other name passed verification. Node's empty servername only stops SNI
from being sent; checkServerIdentity still checks the certificate
against the request's host.
The request now sends no SNI and the verify callback checks the leaf
certificate against the host with mbedTLS's name rules, so a mismatch
fails with ERR_TLS_CERT_ALTNAME_INVALID as it does without servername.
Corpus 5654 covers an empty servername against a certificate for the
host and one for another name, and the SNI names the server receives.
