fix getitem syntax for redis get when value is the empty string

[candeira]

When using __getitem__ syntax to retrieve an empty string, an exception is produced, just as if the key were unset:

>>> r = Redis(**connection_data) >>> print(r.get('bah')) None >>>r['bah'] KeyError: 'bah' >>> r['bah'] = "" >>> r.get('bah') b'' >>> r['bah'] KeyError: 'bah' 

The behaviour should be the same as for any other set value, and the exception only produced for unset keys:

>>> r['bah'] = "" >>> r.get('bah') b'' >>> r['bah'] b'' 

This patch provides a test and a fix.

[andymccurdy]

Thanks!