Fix block loader with missing ignored source (#129006) (#129021)

We miss appending null when ignored_source is not available. Our 
randomized tests already cover this case, but we do not check it when
loading fields.

I labelled this non-issue for an unreleased bug.

Closes #128959
Relates #119546

Author: dnhatn

server/src/main/java/org/elasticsearch/index/mapper/FallbackSyntheticSourceBlockLoader.java

@@ -75,6 +75,7 @@ private record IgnoredSourceRowStrideReader<T>(String fieldName, Reader<T> reade
         public void read(int docId, StoredFields storedFields, Builder builder) throws IOException {
             var ignoredSource = storedFields.storedFields().get(IgnoredSourceFieldMapper.NAME);
             if (ignoredSource == null) {
+                builder.appendNull();
                 return;
             }
 

test/framework/src/main/java/org/elasticsearch/index/mapper/BlockLoaderTestRunner.java

@@ -29,6 +29,8 @@
 
 import static org.apache.lucene.tests.util.LuceneTestCase.newDirectory;
 import static org.apache.lucene.tests.util.LuceneTestCase.random;
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.equalTo;
 
 public class BlockLoaderTestRunner {
     private final BlockLoaderTestCase.Params params;
@@ -74,10 +76,9 @@ private Object load(BlockLoader blockLoader, LeafReaderContext context, MapperSe
         // `columnAtATimeReader` is tried first, we mimic `ValuesSourceReaderOperator`
         var columnAtATimeReader = blockLoader.columnAtATimeReader(context);
         if (columnAtATimeReader != null) {
-            var block = (TestBlock) columnAtATimeReader.read(TestBlock.factory(context.reader().numDocs()), TestBlock.docs(0));
-            if (block.size() == 0) {
-                return null;
-            }
+            BlockLoader.Docs docs = TestBlock.docs(0);
+            var block = (TestBlock) columnAtATimeReader.read(TestBlock.factory(context.reader().numDocs()), docs);
+            assertThat(block.size(), equalTo(1));
             return block.get(0);
         }
 
@@ -99,9 +100,8 @@ private Object load(BlockLoader blockLoader, LeafReaderContext context, MapperSe
         BlockLoader.Builder builder = blockLoader.builder(TestBlock.factory(context.reader().numDocs()), 1);
         blockLoader.rowStrideReader(context).read(0, storedFieldsLoader, builder);
         var block = (TestBlock) builder.build();
-        if (block.size() == 0) {
-            return null;
-        }
+        assertThat(block.size(), equalTo(1));
+
         return block.get(0);
     }