### 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
}
        @Override
        public Query existsQuery(SearchExecutionContext context) {
            return new TermQuery(new Term("_feature", name()));
        }
        @Override
        public IndexFieldData.Builder fielddataBuilder(FieldDataContext fieldDataContext) {
            throw new IllegalArgumentException("[rank_feature] fields do not support sorting, scripting or aggregating");
        }
        @Override
        public ValueFetcher valueFetcher(SearchExecutionContext context, String format) {
            if (format != null) {
                throw new IllegalArgumentException("Field [" + name() + "] of type [" + typeName() + "] doesn't support formats.");
            }
            return new SourceValueFetcher(name(), context) {
                @Override
                protected Float parseSourceValue(Object value) {
                    if (value.equals("")) {
                        return nullValue;
                    }
                    return objectToFloat(value);
                }
            };
        }
        @Override
        public Query termQuery(Object value, SearchExecutionContext context) {
            throw new IllegalArgumentException("Queries on [rank_feature] fields are not supported");
        }
    }
    private final boolean positiveScoreImpact;