Fix dimension values aggregator (elastic#137410)

This change fixes a mistake in the dimension values aggregator. The bug 
prevented the shortcut in production (not correctness issue) and caused
incorrect results in tests when using a block hash that reserves 0 for
nulls. This change corrects the issue and randomly uses two types of
block hash in tests: one that reserves group 0 for nulls and one that
does not.

Closes elastic#137378

Author: dnhatn

muted-tests.yml

@@ -501,9 +501,6 @@ tests:
 - class: org.elasticsearch.readiness.ReadinessClusterIT
   method: testReadinessDuringRestartsNormalOrder
   issue: https://github.com/elastic/elasticsearch/issues/136955
-- class: org.elasticsearch.xpack.esql.expression.function.aggregate.DimensionValuesByteRefGroupingAggregatorFunctionTests
-  method: testSimple
-  issue: https://github.com/elastic/elasticsearch/issues/137378
 - class: org.elasticsearch.xpack.ilm.TimeSeriesDataStreamsIT
   method: testSearchableSnapshotAction
   issue: https://github.com/elastic/elasticsearch/issues/137167

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/aggregation/DimensionValuesByteRefGroupingAggregatorFunction.java

@@ -237,16 +237,17 @@ public void addIntermediateInput(int positionOffset, IntVector groups, Page page
     @Override
     public void evaluateIntermediate(Block[] blocks, int offset, IntVector selected) {
         int positionCount = selected.getPositionCount();
-        boolean allSelected = positionCount == maxGroupId + 1;
+        boolean allSelected = positionCount > maxGroupId;
         if (allSelected) {
             for (int i = 0; i < selected.getPositionCount(); i++) {
-                if (selected.getInt(i) == i) {
+                if (selected.getInt(i) != i) {
                     allSelected = false;
                     break;
                 }
             }
         }
         if (allSelected) {
+            fillNullsUpTo(positionCount);
             blocks[offset] = builder.build();
             return;
         }

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/expression/function/aggregate/DimensionValuesByteRefGroupingAggregatorFunctionTests.java

@@ -77,14 +77,17 @@ public void testSimple() {
             randomFrom(AggregatorMode.INITIAL, AggregatorMode.SINGLE),
             List.of(0)
         );
+        final List<BlockHash.GroupSpec> groupSpecs;
+        if (randomBoolean()) {
+            // Use IntBlockHash; reserves 0 for null values
+            groupSpecs = List.of(new BlockHash.GroupSpec(1, ElementType.INT));
+        } else {
+            // Use PackedValuesBlockHash; does not reserve 0 for null values
+            groupSpecs = List.of(new BlockHash.GroupSpec(1, ElementType.INT), new BlockHash.GroupSpec(1, ElementType.INT));
+        }
         HashAggregationOperator hashAggregationOperator = new HashAggregationOperator(
             List.of(aggregatorFactory),
-            () -> BlockHash.build(
-                List.of(new BlockHash.GroupSpec(1, ElementType.INT)),
-                driverContext.blockFactory(),
-                randomIntBetween(1, 1024),
-                randomBoolean()
-            ),
+            () -> BlockHash.build(groupSpecs, driverContext.blockFactory(), randomIntBetween(1, 1024), randomBoolean()),
             driverContext
         );
         List<Page> outputPages = new ArrayList<>();
@@ -99,7 +102,7 @@ public void testSimple() {
         Map<Integer, List<BytesRef>> actualValues = new HashMap<>();
         for (Page out : outputPages) {
             IntBlock groups = out.getBlock(0);
-            BytesRefBlock valuesBlock = out.getBlock(1);
+            BytesRefBlock valuesBlock = out.getBlock(groupSpecs.size());
             for (int p = 0; p < out.getPositionCount(); p++) {
                 int group = groups.getInt(p);
                 int valueCount = valuesBlock.getValueCount(p);