Description:
------------
when using xmlrpc_encode(), NULL php values are converted into empty xmlrpc string values, which is a pretty sensible decision.

Unfortunately, when using xmlrpc_get_type() to see what type they will be converted into, 'base64' is returned, which is not very coherent.

Reproduce code:
---------------
var_dump(xmlrpc_get_type(null));

Expected result:
----------------
string

Actual result:
--------------
base64