### Description

I wonder if anytime soon you will support the null_value mapping paramater for rank_feature field type. Thanks!
modules/mapper-extras/src/main/java/org/elasticsearch/index/mapper/extras/RankFeatureFieldMapper.java
import org.elasticsearch.index.mapper.FieldMapper;
import org.elasticsearch.index.mapper.MappedFieldType;
import org.elasticsearch.index.mapper.MapperBuilderContext;
import org.elasticsearch.index.mapper.SourceValueFetcher;
import org.elasticsearch.index.mapper.TextSearchInfo;
import org.elasticsearch.index.mapper.ValueFetcher;
import org.elasticsearch.index.query.SearchExecutionContext;
import org.elasticsearch.xcontent.XContentBuilder;
import org.elasticsearch.xcontent.XContentParser.Token;
import java.io.IOException;
import java.util.Map;
import java.util.Objects;
/**
 * A {@link FieldMapper} that exposes Lucene's {@link FeatureField}.
 */
public class RankFeatureFieldMapper extends FieldMapper {
    public static final String CONTENT_TYPE = "rank_feature";
    public static class Defaults {
        public static final FieldType FIELD_TYPE = new FieldType();
        static {
            FIELD_TYPE.setTokenized(false);
            FIELD_TYPE.setIndexOptions(IndexOptions.NONE);
            FIELD_TYPE.setOmitNorms(true);
            FIELD_TYPE.freeze();
        }
    }
    private static RankFeatureFieldType ft(FieldMapper in) {
        return ((RankFeatureFieldMapper) in).fieldType();
    }
    public static class Builder extends FieldMapper.Builder {