### 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
public Builder(String name) {
            super(name);
        }
        Builder nullValue(float nullValue) {
            this.nullValue.setValue(nullValue);
            return this;
        }
        @Override
        protected Parameter<?>[] getParameters() {
            return new Parameter<?>[] { positiveScoreImpact, nullValue, meta };
        }
        @Override
        public RankFeatureFieldMapper build(MapperBuilderContext context) {
            return new RankFeatureFieldMapper(
                name,
                new RankFeatureFieldType(
                    context.buildFullName(name),
                    meta.getValue(),
                    positiveScoreImpact.getValue(),
                    nullValue.getValue()
                ),
                multiFieldsBuilder.build(this, context),
                copyTo.build(),
                positiveScoreImpact.getValue(),
                nullValue.getValue()
            );
        }
    }
    public static final TypeParser PARSER = new TypeParser((n, c) -> new Builder(n));
    public static final class RankFeatureFieldType extends MappedFieldType {