Fix RedisCommand::issue_typed<int>()

[rpj]

The semantics of RedisCommand::issue_typed<int>() are bad, specifically that it can return sentinel integers (INT_MAX - 0x0f or INT_MAX - 0xf0) in the "nil" or error return cases (respectively), meaning that as currently-implemented those values can not be used because they would be ambiguous! This is Bad ™️.

I'm thinking what we really need is an explicit RedisNil subclass of RedisObject that can be used in multiple places (string nil, array nil) and will fit into RESP3 support (#71) as it has an explicit "Nil" value.