Handle the empty label as a literal string.
OpenSSL::SSL::SSLSocket#post_connection_check can raise NoMethodError
when the server certificate contains a malformed subjectAltName
extension with a dNSName that starts with ".", such as ".example.com".
Reported by 4l1ve on HackerOne.
https://github.com/ruby/openssl/commit/1fb29e7ef5