Add coelesce in income query.

accountant/models/accounts.py

@@ -56,17 +56,23 @@ class Account(db.Model):
     def income(self, begin, end):
         """Return the income for a specific time period."""
         query = db.session.query(
+            db.func.coalesce(
                 db.func.sum(
                     Operation.value
                 ).filter(
                     db.func.sign(Operation.value) == -1
+                ), 0
             ).label("expenses"),
+            db.func.coalesce(
                 db.func.sum(
                     Operation.value
                 ).filter(
                     db.func.sign(Operation.value) == 1
+                ), 0
             ).label("revenues"),
-            db.func.sum(Operation.value).label("income")
+            db.func.coalesce(
+                db.func.sum(Operation.value), 0
+            ).label("income")
         ).filter(
             Operation.account_id == self.id,
         )