indexer-alt: use NULL, instead of DEFAULT VALUE

## Description

Add `#[diesel(treat_none_as_default_value = false)]` to model types that
include optional fields. This affects how those fields are written out
to SQL when they contain `None`. Previously (and by default), those
fields would be represented by the keyword `DEFAULT VALUE`, and after
this change, they will be represented by a parameter binding, which will
be bound to `NULL`.

This is semantically identical in our case, because we don't set default
values, but it also results in less variety in prepared statements
(because regardless of the content of fields, they will now all be
represented by a binding), which will improve grouping of statistics
per-statement, and could also improve performance, if those prepared
statements can be cached and re-used.

## Test plan

Re-run indexer on first 100000 checkpoints.

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,
@@ -47,6 +48,7 @@ pub struct StoredFeatureFlag {
 
 #[derive(Insertable, Debug, Clone, FieldCount)]
 #[diesel(table_name = kv_protocol_configs)]
+#[diesel(treat_none_as_default_value = false)]
 pub struct StoredProtocolConfig {
     pub protocol_version: i64,
     pub config_name: String,

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

@@ -15,6 +15,7 @@ use crate::schema::{
 
 #[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,
@@ -70,7 +71,6 @@ pub enum StoredCoinOwnerKind {
 
 #[derive(Insertable, Debug, Clone, FieldCount)]
 #[diesel(table_name = sum_coin_balances, primary_key(object_id))]
-#[diesel(treat_none_as_default_value = false)]
 pub struct StoredSumCoinBalance {
     pub object_id: Vec<u8>,
     pub object_version: i64,
@@ -95,6 +95,7 @@ pub struct StoredSumObjType {
 
 #[derive(Insertable, Debug, Clone)]
 #[diesel(table_name = wal_coin_balances, primary_key(object_id, object_version))]
+#[diesel(treat_none_as_default_value = false)]
 pub struct StoredWalCoinBalance {
     pub object_id: Vec<u8>,
     pub object_version: i64,
@@ -106,6 +107,7 @@ pub struct StoredWalCoinBalance {
 
 #[derive(Insertable, Debug, Clone)]
 #[diesel(table_name = wal_obj_types, primary_key(object_id, object_version))]
+#[diesel(treat_none_as_default_value = false)]
 pub struct StoredWalObjType {
     pub object_id: Vec<u8>,
     pub object_version: i64,