Patched DateField and DateTimeField to check for None values before trying to…

Patched DateField and DateTimeField to check for None values before trying to perform date(time) conversion.

Signed-off-by: Kevin Stone <kevinastone@gmail.com>

rest_framework/fields.py

@@ -534,6 +534,8 @@ class DateField(WritableField):
         raise ValidationError(msg)
 
     def to_native(self, value):
+        if value is None:
+            return None
         if isinstance(value, datetime.datetime):
             value = value.date()
         if self.format.lower() == ISO_8601:
@@ -599,6 +601,8 @@ class DateTimeField(WritableField):
         raise ValidationError(msg)
 
     def to_native(self, value):
+        if value is None:
+            return None
         if self.format.lower() == ISO_8601:
             return value.isoformat()
         return value.strftime(self.format)