Fixed a crash in receipt generation.

matemat/db/facade.py

@@ -647,7 +647,7 @@ class MatematDatabase(object):
                 else:
                     t = Transaction(ta_id, user, value, old_balance, datetime.fromtimestamp(date))
                 transactions.append(t)
-            if write:
+            if write and len(transactions) > 0:
                 cursor.execute('''
                 INSERT INTO receipts (user_id, first_ta_id, last_ta_id)
                   VALUES (:user_id, :first_ta, :last_ta)