Backport of 'Fix empty partition range crashing pipeline lookups (CI outage)' to 19-1

What does this MR do and why?

Backport of !253802 (merged).

Ci::Partitions::SyncService wrote an exclusive range (min...max) into ci_partitions.pipelines_id_range (an int8range column). When a switching partition held exactly one pipeline (min == max), that range was empty and PostgreSQL stored it as 'empty'. PartitionCache#load_from_database only filtered SQL NULL, so the empty row loaded, ActiveRecord cast it to nil, and PartitionCache#store then called nil.begin, raising NoMethodError. This runs inside Ci::Pipeline.find_by_id, so every Sidekiq worker that looks up a pipeline crashed. New pipelines stayed in created and jobs were never sent to runners, an instance-wide CI outage.

The fix uses inclusive ranges (min..max and (max.next..)) so a single-pipeline range is never empty and the max pipeline is not dropped into a gap, and it skips empty ranges when loading the cache (NOT isempty(pipelines_id_range)) to guard against any already-persisted empty range.

Fixes #608165 (closed)

This is a clean cherry-pick of the single fix commit. On 19-1-stable-ee the spec base differs slightly (let_it_be(..., freeze: false) versus let_it_be_with_reload), which the patch does not touch; the applied change is identical to the default-branch MR.

MR acceptance checklist

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
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