### Description

I wonder if anytime soon you will support the null_value mapping paramater for rank_feature field type. Thanks!
modules/mapper-extras/src/test/java/org/elasticsearch/index/mapper/extras/RankFeatureFieldTypeTests.java
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0 and the Server Side Public License, v 1; you may not use this file except
 * in compliance with, at your election, the Elastic License 2.0 or the Server
 * Side Public License, v 1.
 */
package org.elasticsearch.index.mapper.extras;
import org.elasticsearch.index.mapper.FieldTypeTestCase;
import org.elasticsearch.index.mapper.MappedFieldType;
import org.elasticsearch.index.mapper.MapperBuilderContext;
import java.io.IOException;
import java.util.Collections;
import java.util.List;
public class RankFeatureFieldTypeTests extends FieldTypeTestCase {
    public void testIsNotAggregatable() {
        MappedFieldType fieldType = new RankFeatureFieldMapper.RankFeatureFieldType("field", Collections.emptyMap(), true, null);
        assertFalse(fieldType.isAggregatable());
    }
    public void testFetchSourceValue() throws IOException {
        MappedFieldType mapper = new RankFeatureFieldMapper.Builder("field").nullValue(2.0f)
            .build(MapperBuilderContext.root(false))
            .fieldType();
        assertEquals(List.of(3.14f), fetchSourceValue(mapper, 3.14));
        assertEquals(List.of(42.9f), fetchSourceValue(mapper, "42.9"));
        assertEquals(List.of(2.0f), fetchSourceValue(mapper, ""));
    }
}