Allow external plugins to return no value

Allow external plugins to return no value

Instead of using the XML-RPC v2 extension <nil/>, which Perl's
XML::RPC::Parser does not (yet) support (Joey's patch is pending), we
agreed on a sentinel: {'null':''}, that is, a hash with a single key
"null" pointing to the empty string.

The Python proxy automatically converts None appropriately and raises an
exception if a hook function should, by weird coincidence, attempt to
return {'null':''}.

Signed-off-by: martin f. krafft <madduck@madduck.net>