Fix error processing GeoResult

[mgravell]

Investigate and fix #3123; PEBKAC - conversion error with ReadString() vs ReadRedisValue() during the v3 read conversion (0180afa).

OK; so: I've added more notes here in /docs/exp/StringToRedisValue.md - no point me duplicating that all here. Short version: ReadString() was used where ReadRedisValue() was wanted. I have added a trap to prevent this in the future; if I run the build with the original code, I now see:

ResultProcessor.cs(1904, 48): [StringToRedisValue] 'StackExchange.Redis.RedisValue.implicit operator StackExchange.Redis.RedisValue(string?)' is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed. (https://stackexchange.github.io/StackExchange.Redis/exp/StringToRedisValue)

ResultProcessor.cs(1945, 44): [StringToRedisValue] 'StackExchange.Redis.RedisValue.implicit operator StackExchange.Redis.RedisValue(string?)' is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed. (https://stackexchange.github.io/StackExchange.Redis/exp/StringToRedisValue)

Only DEBUG builds are impacted - this trap does not exist in the shipped dll.

Every other usage was a false-positive; I've fixed those by using .AsRedisValue(), but there is no semantic change - just being explicit rather than implicit to tell the build we know what we're doing.