MXS-2650 Fix SSL-use with Connector-C

Authenticators and monitors now use SSL when configured. The fix has two parts:
1) Removed the extra SSLConfig inside SSLProvider, as SSLContext already contains
the config.
2) When inputting parameter values to mysql_ssl_set(), empty strings are converted
to NULL-pointers as the function expects those for unused values.

server/core/server.cc

@@ -527,7 +527,7 @@ void Server::print_to_dcb(DCB* dcb) const
     }
     if (server->ssl().enabled())
     {
-        dcb_printf(dcb, "%s", server->ssl().to_string().c_str());
+        dcb_printf(dcb, "%s", server->ssl().config()->to_string().c_str());
     }
     if (server->proxy_protocol)
     {