fix: use value from currency exchange when exchange api is disabled

[venkat102]

Issue:
exchange rate not fetching from the currency exchange when exchange API setting is disabled.
ref: 32635

Steps to reproduce:

• Create company with Currency AED
• Disable exchange API setting
• Create a currency exchange with From currency USD, To currency AED and exchange rate as 3.67.
• Create a Invoice with party currency as USD
• Exchange rate will be fetched as 3.6725 instead of 3.67.

Before:

exchange-rate-issue.mp4

After:

exchange-rate-fix.mp4

Back port needed for v15