Backport of Fix silent data loss in Advanced Search routing for >128 root_ancestor_ids

What does this MR do and why?

When searching with more than 128 root ancestor IDs, the routing logic in ee/lib/elastic/latest/routing.rb emits { routing: [] } which serializes to routing= (empty string). Elasticsearch interprets this as 'search zero shards', returning 0 hits with HTTP 200 — silent data loss that looks like a successful empty result.

Root cause: The root_ancestor_ids branch lacked the blank guard that the project_ids branch already has. When build_routing returns [] for >128 IDs, it was returned directly as { routing: [] } instead of falling back to an unrouted search.

Fix: Add return {} if routing.blank? to the root_ancestor_ids branch, matching the existing pattern in the project_ids branch. When root ancestor count exceeds 128, the search now degrades gracefully to an unrouted (all-shard) search returning all results.

See merge request !246079 (merged)

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