Fix encoding of None

[cunla]

Fix #117

[github-actions]

Coverage report:

Name                                              Stmts   Miss Branch BrPart  Cover
-----------------------------------------------------------------------------------
fakeredis/__init__.py                                 7      2      0      0    71%
fakeredis/_basefakesocket.py                        213      6     84      7    96%
fakeredis/_command_args_parsing.py                   54      0     32      0   100%
fakeredis/_commands.py                              295      7    110      4    97%
fakeredis/_fakesocket.py                             17      0      0      0   100%
fakeredis/_helpers.py                               167      4     61      3    97%
fakeredis/_msgs.py                                   60      0      0      0   100%
fakeredis/_server.py                                122      6     42      5    93%
fakeredis/_zset.py                                   64      0     16      0   100%
fakeredis/aioredis.py                               155      8     43      7    92%
fakeredis/commands_mixins/__init__.py                 0      0      0      0   100%
fakeredis/commands_mixins/bitmap_mixin.py           116      0     58      0   100%
fakeredis/commands_mixins/connection_mixin.py        21      0      4      0   100%
fakeredis/commands_mixins/generic_mixin.py          212     13     90      8    92%
fakeredis/commands_mixins/hash_mixin.py              80      1     12      1    98%
fakeredis/commands_mixins/list_mixin.py             176      5     76      5    96%
fakeredis/commands_mixins/pubsub_mixin.py            81      0     36      6    95%
fakeredis/commands_mixins/scripting_mixin.py        147      3     70      4    97%
fakeredis/commands_mixins/server_mixin.py            47      0     10      0   100%
fakeredis/commands_mixins/set_mixin.py              139     12     58      6    88%
fakeredis/commands_mixins/sortedset_mixin.py        304      7    121      6    97%
fakeredis/commands_mixins/string_mixin.py           220      5    102      6    97%
fakeredis/commands_mixins/transactions_mixin.py      71      2     20      1    97%
fakeredis/stack/__init__.py                           7      3      0      0    57%
fakeredis/stack/_json_mixin.py                      181      7     79      3    96%
-----------------------------------------------------------------------------------
TOTAL                                              2956     91   1124     72    96%