bug: vertical sharding has issue with absent_over_time

[yeya24]

Describe the bug

    query_fuzz_test.go:169: case 0 results mismatch.
        instant query: quantile without (__name__) (--pi(), absent_over_time({series="2"}[1h:1m]))
        res1: 
        res2: {} => +Inf @[1678391416.275]

While testing vertical sharding with PromQLsmith, I found out that we will hit this inconsistency result between sharding and non sharding Cortex.

The query itself can be simplified to sum by (xxx) (absent_over_time({series="2"}[1h:1m])) and we can still reproduce this issue.
The series matching {series="2"} exist in both Cortex instances.

If vertical sharding enabled, if the shard doesn't contains series matching series=2, absent_over_time will return 1 so causing inconsistency.

[yeya24]

Fixing this on Thanos thanos-io/thanos#6201.