line: omit calculation of balance if credit/debit = None

remove unused imports

line.py

@@ -733,7 +733,12 @@ class Line(SecondCurrencyMixin, Workflow, ModelSQL, ModelView):
 
             lines = Line2.search(query)
             for line3 in lines:
-                balance += getattr(line3, credit_name) - getattr(line3, debit_name)
+                line_credit = getattr(line3, credit_name)
+                line_debit = getattr(line3, debit_name)
+
+                if (line_credit is not None) or (line_debit is not None):
+                    balance += line_credit - line_debit
+
                 if line3.id == line.id:
                     break
             return balance