fix: invalid gain/loss JE created on base currency Expense Claim

(cherry picked from commit 75d95ac)

erpnext/accounts/doctype/payment_entry/payment_entry.py

@@ -809,6 +809,11 @@ def calculate_base_allocated_amount_for_reference(self, d) -> float:
 				flt(d.allocated_amount) * flt(exchange_rate), self.precision("base_paid_amount")
 			)
 
+			# on rare case, when `exchange_rate` is unset, gain/loss amount is incorrectly calculated
+			# for base currency transactions
+			if d.exchange_rate is None:
+				d.exchange_rate = 1
+
 			allocated_amount_in_pe_exchange_rate = flt(
 				flt(d.allocated_amount) * flt(d.exchange_rate), self.precision("base_paid_amount")
 			)