Handle NULLs in hourly_statistics backfill

Use COALESCE for total_fee and total_gas_used when backfilling indexer.hourly_statistics_agg from indexer.hourly_statistics to avoid inserting NULLs. This ensures missing values are treated as 0 during the migration (packages/graphql/database/migrations/046_replace_hourly_statistics_mv_with_agg_table.sql) and preserves the existing ON CONFLICT update behavior.

Author: nelitow

packages/graphql/database/migrations/046_replace_hourly_statistics_mv_with_agg_table.sql

@@ -24,7 +24,7 @@ GRANT SELECT ON indexer.hourly_statistics_agg TO explorer_ro;
 
 -- Backfill from existing MV if it has data
 INSERT INTO indexer.hourly_statistics_agg (hour, total_fee, total_gas_used)
-SELECT hour, total_fee, total_gas_used
+SELECT hour, COALESCE(total_fee, 0), COALESCE(total_gas_used, 0)
 FROM indexer.hourly_statistics
 ON CONFLICT (hour) DO UPDATE
   SET total_fee = excluded.total_fee,