Function to_prodtautclass in diffstrata can return int

The following behaviour is problematic:

sage: from admcycles.diffstrata import *
sage: X = GeneralisedStratum(sig_list=[Signature((5, -4, -1))],res_cond=[[(0, 1)]])
sage: t = (X.xi).to_prodtautclass()
sage: (t, type(t))
(0, <class 'int'>)

Instead of returning a zero prodtautclass, the function above returns an int, which creates error messages when trying to call methods of tautclasses for t further down in the program.