handle null balance trxn id when calculating fees

I had an invoice that is a pro-rated refund of a recurring payment. When I run the Stripe.retryall API, this crashes in CRM_Core_Payment_StripeIPN::setInfo() because we try to set a fee when the $balanceTransactionID is NULL. Since CRM_Core_Payment_Stripe::getFeeFromBalanceTransaction() is clearly set up to handle this scenario (based on the docblock and the if statement) I've changed the function signature to allow a NULL value.

️ PASS: Unit Tests: https://github.com/semperit/CiviCARROT/actions/runs/3108088556

added 2 commits

• 914bcf7f - 1 commit from branch extensions:master
• 51f6eb9c - handle null balance trxn id when calculating fees

Compare with previous version

merged

Thanks @JonGold

FAIL: Unit Tests: https://github.com/semperit/CiviCARROT/actions/runs/3108525564

changed milestone to %6.7