Properly serialize fields when emit_fields option is enabled

[hackermondev]

When the emit_fields option is enabled, it doesn't properly differentiate between default fields (that should be emitted) and optional fields (which should only be emitted if they have a value); there's a mismatch in write_struct_serialize_start and write_serialize_field on whether optional fields are emitted, resulting in an invalid length passed to Serializer::serialize_struct which breaks serialization.

message Host {
    required string host = 1;
    optional agent.http_crawling.ranking.RankingSignals ranking_signals = 2;
    map<string, double> queue_scores = 3;
}

impl serde::Serialize for Host {
    #[allow(deprecated)]
    fn serialize<S>(&self, serializer: S) -> std::result::Result<S::Ok, S::Error>
    where
        S: serde::Serializer,
    {
        use serde::ser::SerializeStruct;
        let mut len = 0;
        if true {
            len += 1;
        }
        if true {
            len += 1;
        }
        if true {
            len += 1;
        }
        let mut struct_ser = serializer.serialize_struct("agent.http_crawling.data.Host", len)?;
        if true {
            struct_ser.serialize_field("host", &self.host)?;
        }
        if let Some(v) = self.ranking_signals.as_ref() {
            struct_ser.serialize_field("rankingSignals", v)?;
        }
        if true {
            struct_ser.serialize_field("queueScores", &self.queue_scores)?;
        }
        struct_ser.end()
    }
}

It only emits rankingSignals if the field has a value, but when calculating the length, it always counts the field.

This change ensures only default fields are emitted when emit_fields is enabled.