[Bug]: Calling API updateTransaction for a sub-transaction clears the category & amount

### Verified issue does not already exist?

- [x] I have searched and found no existing issue

### What happened?

If there is a split transaction like this:


And the following code is run (where `'e070264d-98a9-4c91-b5e0-d4c1a63a219a'` is the id of one of the sub transactions)
```
const api = require('@actual-app/api');
await api.updateTransaction('e070264d-98a9-4c91-b5e0-d4c1a63a219a', {notes: 'New notes'});
```

Then this will be the result:


As you can see, the notes have been updated, but the category & amount has been cleared.

This does not happen for transactions that are not split.

### How can we reproduce the issue?

Repro steps provided above

### Where are you hosting Actual?

Other

### What browsers are you seeing the problem on?

Chrome

### Operating System

Windows 11

Start by tracing the updateTransaction entry point in @actual-app/api using the supplied sub-transaction ID and split-transaction reproduction. Verify the behavior with a split transaction, then confirm that updating only notes preserves the sub-transaction's category and amount.

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