## 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.

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## Release notes

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

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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: