Fix UOE when fetching flattened field

The new fields option allows to fetch the value of all fields in the mapping.
However, internal fields that are used by some field mappers are also shown when
concrete fields retrieved through a pattern (`*` or `foo*`).
We have a [long term plan](elastic#63446) to hide these fields in field_caps and from pattern resolution
so this change is just a hot fix to ensure that they don't break the retrieval in the meantime.
The `flattened._keyed field will show up as an empty field when using a pattern that match the
flattened field.

Relates elastic#63446

x-pack/plugin/mapper-flattened/src/main/java/org/elasticsearch/xpack/flattened/mapper/FlattenedFieldMapper.java

@@ -258,7 +258,7 @@ public IndexFieldData.Builder fielddataBuilder(String fullyQualifiedIndexName, S
 
         @Override
         public ValueFetcher valueFetcher(MapperService mapperService, SearchLookup searchLookup, String format) {
-            throw new UnsupportedOperationException();
+            return lookup -> List.of();
         }
     }
 

x-pack/plugin/mapper-flattened/src/test/java/org/elasticsearch/xpack/flattened/mapper/KeyedFlattenedFieldTypeTests.java

@@ -20,9 +20,11 @@
 import org.elasticsearch.index.mapper.FieldTypeTestCase;
 import org.elasticsearch.xpack.flattened.mapper.FlattenedFieldMapper.KeyedFlattenedFieldType;
 
+import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.Map;
 
 public class KeyedFlattenedFieldTypeTests extends FieldTypeTestCase {
 
@@ -148,4 +150,12 @@ public void testWildcardQuery() {
             () -> ft.wildcardQuery("valu*", null, false, randomMockShardContext()));
         assertEquals("[wildcard] queries are not currently supported on keyed [flattened] fields.", e.getMessage());
     }
+
+    public void testFetchIsEmpty() throws IOException {
+        Map<String, Object> sourceValue = Map.of("key", "value");
+        KeyedFlattenedFieldType ft = createFieldType();
+
+        assertEquals(List.of(), fetchSourceValue(ft, sourceValue));
+        assertEquals(List.of(), fetchSourceValue(ft, null));
+    }
 }