🐛 fix: Skip ES write and refresh when DataFrame is empty

[laurabegin]

Summary

• After the CLIN-5853 indexer refactor, esClient.refreshIndex(currentIndex) fails when df.saveToEs writes nothing because the DataFrame is empty — the index never gets created via es.index.auto.create, so the refresh hits a missing index. Same risk applies to forceMerge, and to createIndex inside the disableReplicas branch.
• Guards the whole post-setupIndex sequence (write, replica toggling, refresh, force merge) behind !df.isEmpty. Empty df is now a true no-op on ES, matching pre-refactor behavior.

Test plan

• CI green
• Manual run against ES with an empty DataFrame (both disableReplicas=true and disableReplicas=false) — confirm no index_not_found_exception
• Manual run with a non-empty DataFrame — confirm write, refresh, and force merge still execute

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