Fix levensthein segmentation fault in invalid name picking

[simonwoerpel]

when the pick function for registry.name receives only "non-name" values, like "-", "(" (which can happen in dirty real world data), the normalized values list is empty which creates (at least on linux, python 3.11) a segmentation fault:

Python 3.11.2 (main, Feb 12 2023, 00:48:52) [GCC 12.2.0]
Type 'copyright', 'credits' or 'license' for more information
IPython 8.10.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: from Levenshtein import setmedian

In [2]: setmedian([])
[1]    307409 segmentation fault  ipython

In this fix we would return None for this case (as seen for other cases in the same function.)