### 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
} else {
            value = context.parser().floatValue();
        }
        if (context.doc().getByKey(name()) != null) {
            throw new IllegalArgumentException(
                "[rank_feature] fields do not support indexing multiple values for the same field [" + name() + "] in the same document"
            );
        }
        if (positiveScoreImpact == false) {
            value = 1 / value;
        }
        context.doc().addWithKey(name(), new FeatureField("_feature", name(), value));
    }
    private static Float objectToFloat(Object value) {
        if (value instanceof Number) {
            return ((Number) value).floatValue();
        } else {
            return Float.parseFloat(value.toString());
        }
    }
    @Override
    protected String contentType() {
        return CONTENT_TYPE;
    }
    @Override
    public FieldMapper.Builder getMergeBuilder() {
        return new Builder(simpleName()).init(this);
    }
}