Currency Rate returns None for Accounting: Bank transactions API when it shows correct value on Xero API Explorer #129

Issue Description:
When using the Xero API Explorer - Accounting API > Bank Transactions, the response returns the expected currency rate. However, when using get_bank_transactions API endpoint, the response returns None for currency rate. This happens to only selected seemingly random cases.

Steps to Reproduce:

1. On Xero API Explorer > Accounting > Bank Transactions, compare the currency_rate for selected period of bank transactions
2. Make a call to the get_bank_transactions endpoint with the required parameters (e.g., xero_tenant_id, xero_status, page, page_size).
3. Inspect the response for currency_rate.

Expected Behavior:
The get_bank_transactions API response should include the correct currency rate as displayed on Xero API Explorer.

Actual Behavior:
The get_bank_transactions API response returns None for currency rate.

API Call:
xero_bank_txn = accounting_api.get_bank_transactions(xero_tenant_id, page)

Sample Output:

{
  ...
  'bank_transaction_id': 'x'xx,
   'contact': None,
   'currency_code': <CurrencyCode.USD: 'USD'>,
   'currency_rate': None,
   'date': datetime.date(2022, 12, 6),
  ...
}

Screenshot:
Xero API Explorer response (expected)

get_bank_transactions response (actual)