locale.getpreferredencoding() should certainly not crash but the
question remains of what should be the outcome. I can see several
possibilities:
(1) return the empty string
(2) return None
(3) return "ascii" (!!)
(4) raise an exception (which one?)

(2) sounds the most logical to me, there is no preferred encoding in the
environment so we just return None to indicate that the application has
to choose its own default.