Closes #5780.

buildUpdateSet computed col.onUpdateFn?.() eagerly for every column, then only used the result when the column was absent from set (set[colName] ?? …). So any $onUpdate callback with side effects — throw, logging, reading external state — ran on every UPDATE, even when the column value was explicitly provided and would win via ??. This is a regression from the lazy 0.44.6 behavior.

const table = pgTable('example', {
  updatedById: uuid('updated_by_id').$onUpdate(() => {
    throw new Error('should not be called when explicitly set');
  }),
  // ...
});

// 0.44.6: ok · 0.45.x: throws
await db.update(table).set({ updatedById: 'some-uuid' }).where(/* … */);

Fix: only evaluate the callback when the column is absent from set (set[colName] == null, matching the existing ?? semantics). The value line is unchanged. Applied to all five dialects that share this code path: pg, mysql, sqlite, singlestore, gel.

Test: added a DB-less unit test (tests/on-update-set.test.ts) — a throwing $onUpdate with an explicit value must not throw, and an absent column must still invoke the callback.

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