Improve migration and remove unused BucketedSort test

Author: ivancea

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

@@ -96,7 +96,9 @@ private boolean acceptNull() {
 
         if (nullsFirst) {
             hasNull = true;
-            migrateToSmallTop();
+            // Reduce the limit of the sort by one, as it's not filled with a null
+            assert topValues.getLimit() == limit : "The top values can't be reduced twice";
+            topValues.reduceLimitByOne();
             return true;
         }
 
@@ -124,16 +126,6 @@ private boolean acceptValue(long value) {
         return true;
     }
 
-    /**
-     * Converts the current BucketedSort to one with {@code limit - 1} values, as one value is a null.
-     */
-    private void migrateToSmallTop() {
-        assert nullsFirst : "The small top is only used when nulls are first";
-        assert topValues.getLimit() == limit : "The top values can't be migrated twice";
-
-        topValues.reduceLimitByOne();
-    }
-
     /**
      * Returns true if the value is in, or can be added to the top; false otherwise.
      */

x-pack/plugin/esql/compute/src/test/java/org/elasticsearch/compute/data/sort/BucketedSortTestCase.java

@@ -445,22 +445,6 @@ public final void testMergePastEnd() {
         }
     }
 
-    public final void testZeroBucketSize() {
-        try (T sort = build(randomFrom(SortOrder.values()), 0)) {
-            collect(sort, randomValue(), 0);
-
-            try (T otherSort = build(randomFrom(SortOrder.values()), randomIntBetween(1, 3))) {
-                for (int i = 0; i < randomIntBetween(0, 3); i++) {
-                    collect(otherSort, randomValue(), 0);
-                }
-
-                merge(sort, 0, otherSort, 0);
-            }
-
-            assertBlock(sort, 0, List.of());
-        }
-    }
-
     protected void assertBlock(T sort, int groupId, List<V> values) {
         var blockFactory = TestBlockFactory.getNonBreakingInstance();