fix: handle edge cases in geographic aggregation

Author: drkaushiksarkar

.github/changes/feature_fix-7868.md

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+# fix: handle edge cases in geographic aggregation
+
+## Summary
+- Fix null handling in district-level rollup
+- Add validation for geographic hierarchy consistency
+- Handle missing population data gracefully
+
+## Test plan
+- [x] Test with missing district