Must initialise empty_tuple under Python 2

Failing to do this meant empty_tuple was still NULL, which caused
PyArg_ParseTupleAndKeywords to fail with an unhelpful message
Python/getargs.c line XX, 'bad argument to internal function'.

Adding specific error messages in Python/getargs.c at this point
(and recompiling Python) quickly identified args==NULL, which I
traced to my non-initialisation of the empty_tuple variable.

This took a while to resolve, as the error was being masked by
the LZMA exceptions.