Fix One field that is required=False, allow_null=True and not provided. to_representation should not explicitly set it to None

[LinanV]

Example

Consider the following serializer and instance:

class MySerializer(serializers.Serializer):
    name = serializers.CharField()
    type = serializers.CharField(required=False, allow_null=True)
    desc = serializers.CharField(required=False, allow_null=True)

instance = {'name': 'example'}
serializer = MySerializer(instance, partial=True)
data = serializer.data
# Current output: {'name': 'example', 'desc': None, 'type': None}
# Proposed output: {'name': 'example'}

Changed

def get_attribute(self, instance
        """
        Given the *outgoing* object instance, return the primitive value
        that should be used for this field.
        """
       ...
           
            # changed
            if not self.required: 
                raise SkipField()
            if self.allow_null:
                return None**
       ...

Description

refs #9458

[auvipy]

your change seems to make the tests fail. would you mind adding tests to verify the change? also in the issue I found some comment that initially discourage this change and you also shared some alternative proposals. care to chime in more? thanks