fix bug on Decimal serialize with null values (#74)

* fix bug on Decimal deserialitzation with null values

* fix 0 case

Author: Jordi Collell

tests/test_schemas.py

@@ -202,13 +202,18 @@ class Guest(typesystem.Schema):
 def test_schema_decimal_serialization():
     class InventoryItem(typesystem.Schema):
         name = typesystem.String()
-        price = typesystem.Decimal(precision="0.01")
+        price = typesystem.Decimal(precision="0.01", allow_null=True)
 
     item = InventoryItem(name="Example", price=123.45)
 
     assert item.price == decimal.Decimal("123.45")
     assert item["price"] == 123.45
 
+    item = InventoryItem(name="test")
+    assert dict(item) == {"name": "test", "price": None}
+    item = InventoryItem(name="test", price=0)
+    assert dict(item) == {"name": "test", "price": 0}
+
 
 def test_schema_uuid_serialization():
     class User(typesystem.Schema):

typesystem/fields.py

@@ -310,7 +310,7 @@ class Decimal(Number):
     numeric_type = decimal.Decimal
 
     def serialize(self, obj: typing.Any) -> typing.Any:
-        return float(obj)
+        return None if obj is None else float(obj)
 
 
 class Boolean(Field):