Crash when using RedisMap::getValue on hash where field does not exist

[bennnjamin]

RedisMap/Hash can return null if the field does not exist in the hash. Just need to change string to ?string

https://github.com/amphp/redis/blob/0b3f38d4f3cfd9d5c576d89d2002f82235c426a8/src/Command/RedisMap.php#L99

Redis Doc: https://redis.io/commands/hget/