The documentation says: 

For *host* and *port*, by passing either an empty string or ``None``,
you can pass ``NULL`` to the C API, so the documentation says it should
work.

This doesn't work because PyString_AsString retruns an empty string, not
NULL. The attached patch adds a check in socketmodule.c to fix this for
host and port.

It's highly debatable whether this is better than fixing the
documentation, though.