### 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
private final Parameter<Boolean> positiveScoreImpact = Parameter.boolParam(
            "positive_score_impact",
            false,
            m -> ft(m).positiveScoreImpact,
            true
        );
        private final Parameter<Map<String, String>> meta = Parameter.metaParam();
        private final Parameter<Float> nullValue = new Parameter<>(
            "null_value",
            false,
            () -> null,
            (n, c, o) -> o == null ? null : objectToFloat(o),
            m -> ft(m).nullValue,
            XContentBuilder::field,
            Objects::toString
        ).addValidator(value -> {
            if (value != null && value < Float.MIN_NORMAL) {
                throw new IllegalArgumentException(
                    "[null_value] must be a positive normal float for field of type [rank_feature], got "
                        + value
                        + " which is less than the minimum positive normal float: "
                        + Float.MIN_NORMAL
                );
            }
        }).acceptsNull();