fix sorting of None values in NMPVCD.do_persist().

[ernstleierzopf]

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Fixes #1314

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Describe changes:

[landauermax]

I do not understand why the lambda expression is necessary here. When there is None in the list, sorting will fail.

>>> x = [b'a', None, b'1']
>>> sorted(list(x), key=lambda L: tuple(el if el is not None else b'-' for el in L))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in <lambda>
TypeError: 'NoneType' object is not iterable

Also, when replacing None with '-' this may be problematic if the value observed in the log lines can also be '-' (which is not unusual, e.g. in Apache Access logs). It is therefore important that None is persisted, and not some string replacement.