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

redis/src/Command/RedisMap.php

Line 99 in 0b3f38d

 public function getValue(string $field): string 

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