Explicit null value serialisation

By default, every Option serialises as an absent field, which is convenient for many cases. Some cases need explicit nulls instead.

Null and absent are not the same to a query. A Core filter or order_by never returns a document that lacks the field, and is_null() matches only a stored null. A pipeline or SQL statement tells them apart too: not_equal and sort keep a document that lacks the field, and SQL IS NULL matches a stored null only. Query and write semantics lists every case.

To help with that there are additional attributes implemented for serde(with):

• For any type:

use serde::{Deserialize, Serialize};
#[derive(Debug, Clone, Deserialize, Serialize)]
struct MyTestStructure {
#[serde(default)]
#[serde(with = "firestore::serialize_as_null")]
test_null: Option<String>,
}

• For Firestore timestamps attribute:

use firestore::*;
use serde::{Deserialize, Serialize};
#[derive(Debug, Clone, Deserialize, Serialize)]
struct MyTestStructure {
#[serde(default)]
#[serde(with = "firestore::serialize_as_null_timestamp")]
test_null: Option<FirestoreInstant>,
}