Fix: SIGSEGV in AggregationFuzzer when reference query returns empty result vector (facebookincubator#17018)

Summary:
Pull Request resolved: facebookincubator#17018

`compareEquivalentPlanResults` calls `mergeRowVectors` on the reference query result without checking if the result vector is empty. When the reference query runner (e.g. SparkQueryRunner) returns a valid `optional` containing an empty `vector<RowVectorPtr>` (query succeeded but produced zero result batches), `mergeRowVectors` crashes accessing `results[0]` on an empty vector.

This was the root cause of recurring Spark Aggregate Fuzzer SIGSEGV crashes in the scheduled CI (e.g. seed 13256, iteration 6).

The fix adds an emptiness check on the result vector before calling
`mergeRowVectors`.

Reviewed By: kgpai

Differential Revision: D99347601

fbshipit-source-id: 9deff0630f7f8e124ffa0a98c9a8b89102fc8acb

Author: pratikpugalia

velox/exec/fuzzer/AggregationFuzzer.cpp

@@ -1079,7 +1079,7 @@ bool AggregationFuzzer::compareEquivalentPlanResults(
               firstPlan, referenceQueryRunner_.get());
           stats_.updateReferenceQueryStats(referenceResult.second);
 
-          if (referenceResult.first) {
+          if (referenceResult.first && !referenceResult.first.value().empty()) {
             velox::fuzzer::ResultOrError expected;
             expected.result = fuzzer::mergeRowVectors(
                 referenceResult.first.value(), pool_.get());