BUG/MINOR: ssl: Don't allow to set NULL sni

BUG/MINOR: ssl: Don't allow to set NULL sni

ssl_sock_set_servername() function was documented to support NULL sni to
unset it. However, the man page of SSL_get_servername() does not mentionned
it is supported or not. And it is in fact not supported by WolfSSL and leads
to a crash if we do so.

For now, this function is never called with a NULL sni, so it better and
safer to forbid this case. Now, if the sni is NULL, the function does
nothing.

This patch could be backported to all stable versions.