NullPointerException in StringValueFacetCounts when using MultiCollectorManager

[shuttie]

Description

When I use MultiCollectorManager which merges both FacetsCollectorManager and TopScoreDocCollectorManager with a query not matching any docs, I expect to get 0 as a facet count, but get NPE:

Cannot read the array length because "this.denseCounts" is null
java.lang.NullPointerException: Cannot read the array length because "this.denseCounts" is null
	at __randomizedtesting.SeedInfo.seed([6C38B1BA02A289D7:8DDEDB50125C0203]:0)
	at org.apache.lucene.facet.StringValueFacetCounts.getTopChildren(StringValueFacetCounts.java:210)
	at org.apache.lucene.facet.TestStringValueFacetCounts.testNarrowSingleValued(TestStringValueFacetCounts.java:102)
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)

A reproducer code:

  public void testNarrowSingleValued() throws Exception {
    Directory dir = newDirectory();
    RandomIndexWriter writer = new RandomIndexWriter(random(), dir);

    Document doc = new Document();
    doc.add(new SortedSetDocValuesField("field", new BytesRef("foo")));
    writer.addDocument(doc);

    IndexSearcher searcher = newSearcher(writer.getReader());
    writer.close();

    TopScoreDocCollectorManager docCollector = new TopScoreDocCollectorManager(10, 10);
    FacetsCollectorManager facetCollector = new FacetsCollectorManager();
    MultiCollectorManager merged = new MultiCollectorManager(docCollector, facetCollector);

    Object[] results = searcher.search(new MatchNoDocsQuery(), merged); // no docs matched!
    StringDocValuesReaderState state =
            new StringDocValuesReaderState(searcher.getIndexReader(), "field");

    StringValueFacetCounts counts = new StringValueFacetCounts(state, (FacetsCollector) results[1]);
    FacetResult top = counts.getTopChildren(10, "field");
    assertEquals(top.childCount, 0); // as no docs matched, it should be zero
    IOUtils.close(searcher.getIndexReader(), dir);
  }

The same code is working flawlessly without NPE on Lucene 9.10, but started throwing NPE on 9.11.

Version and environment details

• Lucene main (from d29c57e), also broken on Lucene 9.11
• Eclipse Temurin JDK 21.0.3
• Linux

[shuttie]

My feeling is that this NPE is related to this change recently backported to 9.11: https://github.com/apache/lucene/pull/12408/files#diff-de1c07b8609beac65af6c1604e2f4aad86f2aa9771615cfb4ce6b93204a7189c

[shuttie]

For the reference, two related parent PRs:

• Backport to 9x: Initialize facet counting data structures lazily #12408 #13300
• Initialize facet counting data structures lazily #12408