Text field block loader properly handles null values from delegate

docs/changelog/127525.yaml

@@ -0,0 +1,5 @@
+pr: 127525
+summary: Text field block loader properly handles null values from delegate
+area: ES|QL
+type: bug
+issues: []

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

@@ -1087,6 +1087,12 @@ public Builder builder(BlockFactory factory, int expectedCount) {
          * using whatever
          */
         private BlockSourceReader.LeafIteratorLookup blockReaderDisiLookup(BlockLoaderContext blContext) {
+            if (isSyntheticSource && syntheticSourceDelegate != null) {
+                // Since we are using synthetic source and a delegate, we can't use this field
+                // to determine if the delegate has values in the document (f.e. handling of `null` is different
+                // between text and keyword).
+                return BlockSourceReader.lookupMatchingAll();
+            }
             if (isIndexed()) {
                 if (getTextSearchInfo().hasNorms()) {
                     return BlockSourceReader.lookupFromNorms(name());

server/src/test/java/org/elasticsearch/index/mapper/blockloader/TextFieldBlockLoaderTests.java

@@ -62,15 +62,8 @@ public static Object expectedValue(Map<String, Object> fieldMapping, Object valu
             if (params.syntheticSource() && testContext.forceFallbackSyntheticSource() == false && usingSyntheticSourceDelegate) {
                 var nullValue = (String) keywordMultiFieldMapping.get("null_value");
 
-                // Due to how TextFieldMapper#blockReaderDisiLookup works this is complicated.
-                // If we are using lookupMatchingAll() then we'll see all docs, generate synthetic source using syntheticSourceDelegate,
-                // parse it and see null_value inside.
-                // But if we are using lookupFromNorms() we will skip the document (since the text field itself does not exist).
-                // Same goes for lookupFromFieldNames().
-                boolean textFieldIndexed = (boolean) fieldMapping.getOrDefault("index", true);
-
                 if (value == null) {
-                    if (textFieldIndexed == false && nullValue != null && nullValue.length() <= (int) ignoreAbove) {
+                    if (nullValue != null && nullValue.length() <= (int) ignoreAbove) {
                         return new BytesRef(nullValue);
                     }
 
@@ -82,12 +75,6 @@ public static Object expectedValue(Map<String, Object> fieldMapping, Object valu
                 }
 
                 var values = (List<String>) value;
-
-                // See note above about TextFieldMapper#blockReaderDisiLookup.
-                if (textFieldIndexed && values.stream().allMatch(Objects::isNull)) {
-                    return null;
-                }
-
                 var indexed = values.stream()
                     .map(s -> s == null ? nullValue : s)
                     .filter(Objects::nonNull)