Fix toXContent / parsing

Author: carlosdelest

server/src/main/java/org/elasticsearch/search/vectors/KnnVectorQueryBuilder.java

@@ -105,7 +105,7 @@ public class KnnVectorQueryBuilder extends AbstractQueryBuilder<KnnVectorQueryBu
             optionalConstructorArg(),
             (p, c) -> RescoreVectorBuilder.fromXContent(p),
             RESCORE_FIELD,
-            ObjectParser.ValueType.OBJECT_OR_NULL
+            ObjectParser.ValueType.OBJECT
         );
         PARSER.declareFieldArray(
             KnnVectorQueryBuilder::addFilterQueries,
@@ -406,9 +406,7 @@ protected void doXContent(XContentBuilder builder, Params params) throws IOExcep
             builder.endArray();
         }
         if (rescoreVectorBuilder != null) {
-            builder.startObject(RESCORE_FIELD.getPreferredName());
-            rescoreVectorBuilder.toXContent(builder, params);
-            builder.endObject();
+            builder.field(RESCORE_FIELD.getPreferredName(), rescoreVectorBuilder);
         }
         boostAndQueryNameToXContent(builder);
         builder.endObject();