indexer-alt: treat None as NULL (#20704)

## Description

Pick out this change from the bigger #20482. `DEFAULT VALUE` is not
something we use, and treating `None` as `DEFAULT VALUE` means we have
to prepare different statements for each insert/update based on where
the `None`'s appear.

By treating `None` as `NULL`, we get to re-use prepared statements more,
and the query sampler will do a better job grouping similar
inserts/updates.

## Test plan

Run indexer on coin balances and object info, locally, for the first
10,000 checkpoints.

---

## Release notes

Check each box that your changes affect. If none of the boxes relate to
your changes, release notes aren't required.

For each box you select, include information after the relevant heading
that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] Indexer: 
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [ ] CLI: 
- [ ] Rust SDK:
- [ ] REST API:

crates/sui-indexer-alt-schema/src/epochs.rs

@@ -8,6 +8,7 @@ use crate::schema::{kv_epoch_ends, kv_epoch_starts, kv_feature_flags, kv_protoco
 
 #[derive(Insertable, Debug, Clone, FieldCount)]
 #[diesel(table_name = kv_epoch_ends)]
+#[diesel(treat_none_as_default_value = false)]
 pub struct StoredEpochEnd {
     pub epoch: i64,
     pub cp_hi: i64,

crates/sui-indexer-alt-schema/src/objects.rs

@@ -14,6 +14,7 @@ use crate::schema::{coin_balance_buckets, kv_objects, obj_info, obj_versions};
 
 #[derive(Insertable, Debug, Clone, FieldCount)]
 #[diesel(table_name = kv_objects, primary_key(object_id, object_version))]
+#[diesel(treat_none_as_default_value = false)]
 pub struct StoredObject {
     pub object_id: Vec<u8>,
     pub object_version: i64,
@@ -49,6 +50,7 @@ pub enum StoredCoinOwnerKind {
 
 #[derive(Insertable, Debug, Clone, FieldCount)]
 #[diesel(table_name = obj_info, primary_key(object_id, cp_sequence_number))]
+#[diesel(treat_none_as_default_value = false)]
 pub struct StoredObjInfo {
     pub object_id: Vec<u8>,
     pub cp_sequence_number: i64,
@@ -62,6 +64,7 @@ pub struct StoredObjInfo {
 
 #[derive(Insertable, Debug, Clone, FieldCount)]
 #[diesel(table_name = coin_balance_buckets, primary_key(object_id, cp_sequence_number))]
+#[diesel(treat_none_as_default_value = false)]
 pub struct StoredCoinBalanceBucket {
     pub object_id: Vec<u8>,
     pub cp_sequence_number: i64,