Empty dataframe should signal a cache miss
Follow-up for https://gitlab.com/gitlab-com/gl-infra/capacity-planning-trackers/gitlab-com/-/issues/1675#note_1866794342:
The underlying reason for the lack of historical data here is that we stored empty dataframes in the cache. These currently don't lead to querying more data from lower level caches in the cache hierarchy and also not from Prometheus. Hence this stops data retrieval, where it shouldn't.
The fix here is to treat empty dataframes as a cache-miss instead of valid data.
We may want to backfill the missing data. This is tricky though because Thanos downsamples this, so today we can only query 6h resolution data. Tamland by default queries 1h resolution data, so it won't get any data back from Thanos.