changed unit default in ingredient parser to none

because empty name units are not accepted by unit serializer but null values can be handled by the ingredient serializer (as a unit can be null)

cookbook/helper/ingredient_parser.py

@@ -203,7 +203,7 @@ class IngredientParser:
     def parse(self, x):
         # initialize default values
         amount = 0
-        unit = ''
+        unit = None
         ingredient = ''
         note = ''
         unit_note = ''