Fix TypeError by handling NoneType in set_print_format_fields

[nelsonmpanju]

##Details
This pull request fixes a TypeError in the set_print_format_fields method of the Journal Entry doctype. The error occurs when attempting to add a float to a NoneType. The changes ensure that NoneType values are handled properly by using the flt function with a default value.

##Problem
When running the Payment Reconciliation process, a TypeError is thrown due to an attempt to add a NoneType to a float. This occurs when either debit_in_account_currency or credit_in_account_currency is None.

##Solution
The solution involves updating the set_print_format_fields method to use the flt function with a default value of 0.0, ensuring that NoneType values are converted to 0.0.

##Changes Made
Modified the set_print_format_fields method in journal_entry.py to handle NoneType values properly.
Updated the calculation logic to use flt(d.debit_in_account_currency, 0.0) + flt(d.credit_in_account_currency, 0.0).