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/modules/routing/binlogrouter/blr.cc

@@ -1481,7 +1481,7 @@ static void diagnostics(MXS_ROUTER* router, DCB* dcb)
 
     if (ssl.enabled())
     {
-        dcb_printf(dcb, "%s", ssl.to_string().c_str());
+        dcb_printf(dcb, "%s", ssl.config()->to_string().c_str());
     }
 
     /* Binlog Encryption options */