No params for xmlrpc() causes PHP 5.3.x failure of XML-RPC

In PHP 5.1.6, this doesn't happen. I don't know what version association.drupal.org is running, but it doesn't happen there either. However, in PHP 5.3.1, the checking of $param_arr in call_user_func_array() is stricter. At it's simplest, here's an example of where it goes wrong (which is the point of this report):

xmlrpc('http://www.example.com/xmlrpc.php', 'system.listMethods');

The unlisted third argument there is the beginning of the $args that will eventually, in xmlrpc_server_call(), cause the following error in PHP 5.3.1: call_user_func_array() expects parameter 2 to be array, null given in includes/xmlrpcs.inc on line 209. To fix this, we would either need to a) ensure that all coded xmlrpc() calls always pass in an empty array() for the third argument or b) default to an empty array (instead of NULL) in all arg-less requests made to the XML-RPC server.

There are actually two errors that are caused whenever an arg-less request is made - the one above, but also: Undefined property: stdClass::$params in includes/xmlrpcs.inc on line 68., which references this usage:

$result = xmlrpc_server_call($xmlrpc_server, $xmlrpc_server->message->methodname, $xmlrpc_server->message->params);

When an XML-RPC client requests a method that has no argument (such as system.listMethods), then $xmlrpc_server->message->params is never set, and the above error triggers. Params are only ever set (via the XML parser kicked off by xml_message_parse(); see also xmlrpc_message_tag_close()) when there are actually params passed through. If there are no params, it's never defaulted to anything, eventually becoming the "undefined property" in the message above. This "undefined property" becomes NULL, gets passed to xmlrpc_server_call() and then causes the error about call_user_func_array().

The attached patch causes the server to (liberally) accept XML with no params, and defaults $xmlrpc_server->message->params to an array if no values are passed through. We DO want to use isset() here, because a param of '' or 0 should be perfectly valid.

Affects both 6 and 7; patches attached for both.

[NOTE: This is my first core patch in *years*. I will fail. Newb me.]