Fix passing empty sequence to max

Previously, when there were no Tasks and `Tasks.generate_id` was called, the `max` function would throw a ValueError.

Now we check to see if the Task list is empty before generating a new id.

calcure/data.py

@@ -297,6 +297,8 @@ def move_task(self, number_from, number_to):
 
     def generate_id(self):
         """Generate a id for a new item. The id is generated as maximum of existing ids plus one"""
+        if self.is_empty():
+            return 0
         return max([item.item_id for item in self.items]) + 1