[ESQL] Shortcircuit grouping aggs on null value (elastic#129708)

Minor aggregators change plus a test to ensure we check that code path.

We were doing a `value.isNull()` for each group the value had to be aggregated in, instead of doing it just once at the beginning.

The affectation goes as follows:
- 1 group, null value: We avoid some extra lightweight calls
- N groups, null value: We avoid calling it N times
- No groups, or non-null value: No effect

Author: ivancea

x-pack/plugin/esql/compute/gen/src/main/java/org/elasticsearch/compute/gen/GroupingAggregatorImplementer.java

@@ -22,6 +22,7 @@
 
 import java.util.Arrays;
 import java.util.List;
+import java.util.Objects;
 import java.util.function.Consumer;
 import java.util.function.Function;
 import java.util.stream.Collectors;
@@ -411,10 +412,16 @@ private MethodSpec addRawInputLoop(TypeName groupsType, TypeName valuesType) {
 
         builder.beginControlFlow("for (int groupPosition = 0; groupPosition < groups.getPositionCount(); groupPosition++)");
         {
-            if (groupsIsBlock) {
-                builder.beginControlFlow("if (groups.isNull(groupPosition))");
+            if (groupsIsBlock || valuesIsBlock) {
+                String conditions = Stream.of(
+                    groupsIsBlock ? "groups.isNull(groupPosition)" : null,
+                    valuesIsBlock ? "values.isNull(groupPosition + positionOffset)" : null
+                ).filter(Objects::nonNull).collect(Collectors.joining(" || "));
+                builder.beginControlFlow("if (" + conditions + ")");
                 builder.addStatement("continue");
                 builder.endControlFlow();
+            }
+            if (groupsIsBlock) {
                 builder.addStatement("int groupStart = groups.getFirstValueIndex(groupPosition)");
                 builder.addStatement("int groupEnd = groupStart + groups.getValueCount(groupPosition)");
                 builder.beginControlFlow("for (int g = groupStart; g < groupEnd; g++)");
@@ -430,9 +437,6 @@ private MethodSpec addRawInputLoop(TypeName groupsType, TypeName valuesType) {
             }
 
             if (valuesIsBlock) {
-                builder.beginControlFlow("if (values.isNull(groupPosition + positionOffset))");
-                builder.addStatement("continue");
-                builder.endControlFlow();
                 builder.addStatement("int valuesStart = values.getFirstValueIndex(groupPosition + positionOffset)");
                 builder.addStatement("int valuesEnd = valuesStart + values.getValueCount(groupPosition + positionOffset)");
                 if (aggParam.isArray()) {