[Bug] converted amount in transaction details can be NULL in some cases

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Describe the issue

In `netsuite2__transaction_details` the column `converted_amount` can be `NULL`, while the `transaction_amount` column is not empty. I guess this has to do with the way an exchange rate is determined in your code.

Our default currency is `EUR`. I have taken a look at the code that was used to determine the correct exchange rate but did not fully comprehend the logic behind it. The expected output for `converted_amount` when the currency is our default currency would be that the exchange rate would be `1`.

### Relevant error log or model output

```shell
Ran query:

select transaction_id , transaction_line_id , transaction_number, transaction_status, account_id , account_number , currency_symbol , converted_amount , transaction_amount , is_main_line , is_tax_line
from transformations.netsuite2__transaction_details
where converted_amount is null
;

And this results in 719 rows, where the other 2000 rows all have a converted_amount as expected.
```

### Expected behavior

The column `netsuite2__transaction_details.converted_amount` should never be NULL.

### dbt Project configurations

Don't think it is relevant here, but this is what we have configured:

```yml
....

vars:
netsuite_data_model: netsuite2
netsuite_schema: netsuite_suiteanalytics
netsuite2__using_vendor_categories: false
netsuite2__using_jobs: false

balance_sheet_transaction_detail_columns: ['bq_contract_id', 'amount_excl_vat', 'transaction_type']
income_statement_transaction_detail_columns: ['bq_contract_id', 'amount_excl_vat', 'transaction_type']

transactions_pass_through_columns:
- name: "transaction_due_on"
transform_sql: "cast(duedate at time zone 'UTC' as date)"
- name: "transaction_on"
transform_sql: "cast(trandate at time zone 'UTC' as date)"
- name: "transaction_closed_on"
transform_sql: "cast(closedate at time zone 'UTC' as date)"
transaction_lines_pass_through_columns:
- name: "custcol_bq_contract"
alias: "bq_contract_id"
- name: "foreignamount"
alias: "amount_excl_vat"
...
models:
netsuite_source:
+schema: base
netsuite:
+schema: transformations
```

### Package versions

Basicly I'm using `0.12.0`, but due to https://github.com/fivetran/dbt_netsuite/issues/113 I started using the following in my `packages.yml`:

```yml
packages:
- git: "https://github.com/FrankTub/dbt_netsuite"
revision: feature/transaction-line-fields
```

### What database are you using dbt with?

postgres

### dbt Version

1.7.11

### Additional Context

Example output of my query:
[netsuite2__transaction_details_202404051706.csv](https://github.com/fivetran/dbt_netsuite/files/14886861/netsuite2__transaction_details_202404051706.csv)

### Are you willing to open a PR to help address this issue?

- [ ] Yes.
- [X] Yes, but I will need assistance and will schedule time during our [office hours](https://calendly.com/fivetran-solutions-team/fivetran-solutions-team-office-hours) for guidance
- [ ] No.