Fix NPE on server startup when transactions table is empty

[fivestones]

Problem

When starting the server against a fresh, empty database, the application enters a crash loop.

The instant.db.model.transaction/init function queries MAX(id) from the transactions table. On a fresh install, this table is empty, so Postgres returns NULL. The code attempts to cast this nil result to a primitive long via (long ...), resulting in a NullPointerException.

Because of this the server crashes and instantdb doesn't work at all.

Context

This appears to be a regression introduced in #2107. That PR added logic to populate the transaction ID on initialization but does not handle the case where the database is completely empty (returning nil).

Solution

Updated max-tx-id in src/instant/db/model/transaction.clj to handle the nil case by defaulting to 0.

Steps to Reproduce

(Assumes you are starting with a fresh install and there is not yet any database)

1. git clone https://github.com/instantdb/instant.git
2. Navigate to instant/server/.
3. Run make docker-compose.
4. Observe the crash in the logs during the init.finish.tx-model phase.

[stopachka]

Thank you @fivestones !