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!