* Many callbacks are optional parameters yet we were asserting on
  Py_None
* We now check to see if allow_none is set when setting up callbacks,
  if it is set and py_function == Py_None, we set the closure to NULL
  and return
* pygi-invoke.c now checks to see if the closure == NULL when setting
  arguments
* if it is NULL there is no reason to set the the destroy notify handler
  so we skip that too