### 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 Float nullValue;
    private RankFeatureFieldMapper(
        String simpleName,
        MappedFieldType mappedFieldType,
        MultiFields multiFields,
        CopyTo copyTo,
        boolean positiveScoreImpact,
        Float nullValue
    ) {
        super(simpleName, mappedFieldType, multiFields, copyTo, false, null);
        this.positiveScoreImpact = positiveScoreImpact;
        this.nullValue = nullValue;
    }
    @Override
    public Map<String, NamedAnalyzer> indexAnalyzers() {
        return Map.of(mappedFieldType.name(), Lucene.KEYWORD_ANALYZER);
    }
    @Override
    public RankFeatureFieldType fieldType() {
        return (RankFeatureFieldType) super.fieldType();
    }
    @Override
    protected void parseCreateField(DocumentParserContext context) throws IOException {
        float value;
        if (context.parser().currentToken() == Token.VALUE_NULL) {
            if (nullValue == null) {
                
                return;
            }
            value = nullValue;