### 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 boolean positiveScoreImpact;
        private final Float nullValue;
        public RankFeatureFieldType(String name, Map<String, String> meta, boolean positiveScoreImpact, Float nullValue) {
            super(name, true, false, false, TextSearchInfo.NONE, meta);
            this.positiveScoreImpact = positiveScoreImpact;
            this.nullValue = nullValue;
        }
        @Override
        public String typeName() {
            return CONTENT_TYPE;
        }
        public boolean positiveScoreImpact() {
            return positiveScoreImpact;